Whether your retirement is looming on the horizon or many decades away, it's always a good time to be planning for a more financially free future. Constructing a robust, long-term investment portfolio is a key element of building out a sustainable and profitable retirement plan.

If you've set a retirement goal of $1 million and you currently have $50,000 to invest, here are three unstoppable stocks to consider allocating part or all of this amount to as work toward your overarching long-term portfolio goals. 

1. Doximity 

Doximity's (DOCS 2.21%) platform provides a range of services designed to make the lives of healthcare professionals easier. The company's platform has been adopted by more than 80% of doctors in the U.S. and over half of all nurse practitioners and physician assistants. 

Doximity's massive professional platform allows healthcare workers to do everything, from connecting with colleagues in a HIPAA-compliant format to perusing and applying for job listings in their chosen specialty to networking with other clinicians across the country.

Currently, Doximity makes most of its revenue from various types of subscription fees. These are derived from marketing solutions, which are essentially fees paid by pharmaceutical companies and other healthcare entities that advertise to Doximity's users, as well as hiring solutions, where its platform allows recruiters to connect with medical professionals across the country. The company also makes money from other growing business areas like its telehealth platform.

In the most recent quarter, Doximity delivered total revenue of $102 million, a 30% increase year over year. It also reported a net income of $26.3 million and generated a free cash flow of nearly $40 million. Over the trailing 12 months, the company has grown its revenue and net income by 86% and 181%, respectively. Meanwhile, analysts are estimating that Doximity can grow its revenue and earnings per share by respective percentages of 40% and 98% over the next two fiscal years alone.   

Doximity is only in its very early stages of growth. Its products and services meet essential needs facing healthcare professionals on a daily basis, and the valuable ad space it sells to healthcare companies is a notable commodity as businesses across the industry rise to meet the demands of the digital age. This could continue to drive durable top and bottom-line growth forward, and foreseeably multiply the healthcare stock's returns for investors many times over the next decade. 

2. Airbnb 

As more and more people seek flexible ways to live and work, capitalizing on the growth of the digital economy, Airbnb's (ABNB 3.21%) platform can also benefit from the changing needs of the modern consumer, as well as the broader tailwinds driving the travel space. 

There's no doubt that the ongoing travel recovery has spurred Airbnb's business forward, but this is far from the only catalyst driving the platform's phenomenal growth. It's becoming increasingly clear that people are using Airbnbs, not just for short-term vacation stays, but to actually live in. Currently, one-fifth of all bookings taking place on the platform are long-term stays of 28 days or more.  

The company just reported its most profitable quarter ever with a net income of $1.2 billion. Meanwhile, its revenue grew nearly 30% percent year over year, to $2.9 billion, in the third quarter of 2022. Airbnb also saw nearly 100 million nights and experiences booked on its platform in the three months alone, a 25% increase from the year-ago period.

While some might be tempted to attribute these year-over-year comparisons to "revenge travel", these are impressive stand-alone growth figures compared to pre-pandemic numbers. Case in point: revenue and net income in the third quarter were up by approximately 70% and 260%, respectively, compared to the same period in 2019. 

A recession could impact spending across all sectors, and travel is certainly one of them. However, Airbnb is deriving its growth from a variety of tailwinds, which gives it the option to grow at a different pace than the average travel-oriented stock, even in a recessionary environment. And, the company's recent foray into traditional apartment rentals shows that travel is only one slice of management's long-term vision for the platform.

Airbnb's diversified business model and compelling growth story make the business ideal for a years-long buy-and-hold investment. Some Wall Street analysts estimate that the stock could soar by nearly 90% in the next 12 months alone. Over the longer term, the Street estimates an impressive 20% annual earnings growth. It's not remotely a stretch of the imagination to think that the stock could outpace that growth rate several times over in the coming decade and provide superb portfolio tailwinds for investors working toward a $1 million retirement goal.  

3. Upstart 

Upstart Holdings' (UPST 1.31%) mission to shake up the world of lending and open up access to credit to an entirely new swath of consumers isn't a foregone conclusion. The stock has tanked more than 90% percent over the last year as investors have grown increasingly doubtful about Upstart's ability to achieve this mission and come out on the other side of a recessionary environment. 

However, a closer look at Upstart's underlying business reveals a far less bleak picture that could prove to be an opportunity for risk-resilient, long-term investors. Management laid out the key catalysts behind its top- and bottom-line declines in the third-quarter earnings call. Here's what CEO Dave Girouard said:

Higher interest rates and significantly elevated risk in the economy means we're approving about 40% fewer applicants than we would have a year ago. And those approved today are seeing offers about 800 basis points higher than they would have a year ago. This accounts for the vast majority of the reduction in volume. 

Management has said that the company is "continually calibrating our risk models to the market". While loan volumes are down right now, and therefore impacting both Upstart's revenue growth and profitability, this is actually a sign that its proprietary underwriting model, which leverages nontraditional data points and artificial intelligence to assess applicants' credit worthiness, is working exactly as it ought to.

Many economists are targeting the beginning of 2024 as the inflection point at which the macro environment could begin to make a decisive return to normal. However, as the economy climbs over its slump in the next few years, Upstart's rapidly evolving and proven model portends well for its recovery and growth against the much broader context of the coming decade. Consensus Street estimates show a solid 30% annual earnings growth over the next five years. With a forward 12-month price-to-earnings ratio of just 10.6, the market doesn't seem to factor in Upstart's long-term growth potential. 

All in their early-growth stages

The beauty of these businesses is that all three are in their early growth phases and have been extremely successful in implementing their original business ideas. That raises the real possibility of further innovations in the pipeline the market is still unaware of. Think Amazon and Meta (formerly Facebook) in their early days. Who knew they would venture into the hugely profitable cloud computing and Instagram businesses, respectively, over and above their originally successful businesses of e-commerce and social media? Doximity, Airbnb, and Upstart Holdings, all hold promises of further game-changing innovations, and that's what is exciting about them. Together, these three stocks have the potential for multi-bagger returns over the next decade.