I doubt that. At some point (between 2010-2012), SXM got accepted as a standard feature in most new car radios. Then it took a few more years for the majority of people to get a new car. So now most of the country has access to SXM, and the subs have hit 27-28mil. And while there are still some people who are driving older cars without built-in access yet, few of those will start subscribing when they eventually do get a new car, because they've been making due without it for so long.
But as far as growth, SXM has zero marketing, and the availability of wifi/cellular streaming services improves everyday. Plus, streaming is something that people are already doing and familiar with, so when it becomes available in their car, they'll already be subscribed to the services. There is literally nothing new on the horizon for SXM, and I'm sure the satellites will be retired in the next few years, because they'll need maintenance/upgrades, and there's not enough of a market to support the massive expense of doing that.