At Disney's (NYSE:DIS) investor day last April, management laid out expectations to grow its soon-to-launch flagship streaming service from zero to 60 million-90 million global subscribers within five years of its debut. The day after Disney+ launched in November, the company said it attracted 10 million sign-ups (although some of those sign-ups came well before the launch).

All indications are that Disney+ has continued growing its subscriber base in the seven weeks since it became available. App downloads totaled about 22 million in the four weeks following the launch, according to data from Apptopia. And a recent survey from Rosenblatt Securities analyst Bernie McTernan found 57% of respondents that subscribe to streaming video services include Disney+ in their channel lineup. That led McTernan to estimate Disney+ already has 25 million subscribers.

At least one analyst thinks Disney likely underestimated the global demand for Disney+. And the rapid adoption of the service early on suggests that's likely the case. To be sure, Disney+ is already showing signs of being a transformational success for the century-old media company. But there are a couple reasons to temper excitement about its potential despite its early success.

Kevin Mayer on stage in front of a Disney+ logo.

Kevin Mayer, head of Disney's direct-to-consumer and international division. Image source: Disney.

If you didn't know about Disney+ before, you do now

At the investor day in April, management laid out plans for its marketing campaign behind Disney+. Its goal was to reach 95% of its target audience by the time the service launched in November.

When Rosenblatt conducted a survey of streaming service subscribers in October, however, just 63% of respondents said they'd heard of Disney+. But when it conducted the survey again at the end of December, that number climbed to 96% -- right on target.

Increased awareness has come as a result of massive advertising spend by Disney, as well as a little luck (who saw Baby Yoda coming?). But Disney has taken every opportunity to push its flagship streaming service -- online, via billboards, in its parks, and through pre-roll trailers in theaters. With several box-office hits since Disney+'s launch, it's no wonder awareness has climbed to near ubiquity.

But now that everyone knows about Disney+, the low-hanging fruit is taken. Going forward, Disney will have to adopt Netflix's (NASDAQ:NFLX) marketing approach. Netflix also has practically 100% brand awareness, so it's moved to marketing awareness for its original and exclusive series in order to draw in new subscribers. That marketing is effective, but considerably more expensive than simple brand awareness ad campaigns.

Disney+ is easily understood

Where Disney+ truly excels, especially compared to its competition, is that it's easily understood. And Disney's marketing emphasizes the simplicity of its product. "Disney + Pixar + Marvel + Star Wars + National Geographic" is emblazoned below the Disney+ logo on most advertisements, as Matthew Ball points out. These are all well-understood brands, and everyone in Disney's target audience knows them and their productions extremely well.

When combined with high brand awareness, the easily understood nature of Disney+ makes the decision simple for consumers. They already know whether they want it, and they know whether it's worth $7 per month or $70 per year to them. As a result, the pace of Disney+ signups will quickly slow because it's a relatively easy decision.

The surprising part is that a lot of consumers have decided they want Disney+. Demand among households with children is high (as expected), with 73% of households with children saying they subscribe to the nascent streaming service. But 43% of households without children also signed up for the service, according to Rosenblatt's survey. That's led to much higher signups in the early going than either Disney or analysts were expecting.

McTernan expects Disney+ to end its first year with about 39 million subscribers, indicating a stark slowdown in growth over the next nine months or so. Still, that would put the service well ahead of management's outlook, and make it an undeniable success for Disney. For reference, Netflix added just 28 million subscribers over the past 12 months (albeit at nearly twice the average price).

Strong early adoption is great for Disney and the broader-than-anticipated appeal of the service is a surprise that could push total subscribers beyond management's expectations. Still, growth going forward won't come nearly as easily as it did in the early weeks.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.