The Strava guys addressed that in their statement, I think. They said that they've tried ads in the feed and product promotions, and got pushback from users each time. You have to give tehm credit for maintaining such an ad-free social media platform in this day and age; compared to my Bookface or instagram feeds the lack of people trying to sell me stuff is quite refreshing on Strava.

I hope they make a subscription model work, but I'm personally not signing up any time soon.