He's not shrinking by moving to one of the most ubiquitous apps in the world. You're just going to see some brief internet rage before people type "rogan" into the Spotify app they already have running all day.
That is nothing like XM radio where I've never met someone who said they had it.
I'm a casual watcher of his show (maybe 1 show every few weeks). Spotify has made no investment into their platform to support podcasts. No notification system for new episodes, no already listened to tracking, no subscriptions or an area where i can find all my podcasts.
I like the idea of having my music and podcasts all in one place which working across all my devices (Linux, iOS, Windows) and i gave up on them. Spotify may be ubiquitous but they're not engineered for podcasts.
That is nothing like XM radio where I've never met someone who said they had it.