Source: Sirius XM

Sirius XM Holdings (NASDAQ:SIRI) has feasted on the back of car sales over the years, so it's only natural to wonder what would happen if auto sales were to shift into reverse.

Some will argue that it's already happening. The popularity of mass transit, next-gen car services, and the urbanization movement are giving drivers fewer reasons to own a car.  

KPMG's Gary Silberg was on CNBC on Tuesday morning, discussing a study that shows the trend away from homes that own two cars or more. The study concludes that just 43% of homes will have two cars or more come 2040, down from today's household rate of 57%. 

The demographic shift is real, and for now it's being led largely by millennials who don't want to spend roughly $5,000 per year in average auto ownership costs when there are more cost-effective ways to get around. With money tied to student loan payments, smartphone data plans, and online connectivity, it just doesn't make a lot of sense to own a car in major cities where Zipcar-like cars are available by the hour and mass transit is reasonable. Commuters are taking to the bus and train more than they used to with mass transit usage up 37% since 1995.

However, the real game changer these days is the booming popularity of peer-to-peer taxi services. Uber's leading the way with others including Lift as alternatives to folks who need a quick and cheap ride. Uber's success -- and with its latest financing round valuing the upstart at a cool $30 billion, we can unequivocally call it a success -- has to leave the automotive industry scratching their heads. 

Driving a car is becoming a cottage industry, and that opens the door for more active drivers who will need to upgrade more often. However, it will also naturally limit the number of people buying cars, and that's where Sirius XM investors need to be careful.

Growth itself has been slowing. Sirius XM's top-line growth has gone from 13% in 2012 to 12% last year. Its guidance suggests revenue growth of 9% this year, and analysts see that continuing to decelerate to 8% come 2015. This growth has been riding on Sirius XM's receivers penetrating the car market, but what happens if Uber continues to take off?

KPMG's Silberg pointed out on CNBC that cars sit dormant 90% of the time. This is the reason why car-sharing platform RelayRides and now the Uber-led peer-to-peer driving service revolutions have taken off. Sirius XM's been successful despite its in-car receivers being inaccessible most of the day (customers pay extra for streaming access). If more people decide to ditch the car and take advantage of Uber to get around it could be a problem. 

Even if the Uber driver offers Sirius or XM to passengers, it's all about lowering the number of cars needed in an asset-sharing model. Then again, now that Spotify struck a deal to let passengers stream their own playlists in select cities, maybe even assuming that Uber drivers will have satellite radio subscriptions could be a stretch.

Sirius XM isn't going away. It's big. It's profitable. It has a monopoly on satellite radio. However, the Uber trend could be challenging as more consumers approach auto ownership differently. Sirius XM survived the threat of streaming services, but now it will have to overcome the potentially more problematic movement away from cars themselves.