I subscribe to a couple of shows which offer free versions, discoverable through Apple's directory and others, and paid "upgrades". Those paid upgrades come into the same app I use for listening to podcasts generally. I actually think things today are pretty well set up for podcasters who want to be independent but still get paid.
I write fiction on the side and am able to put my ebooks (and print books, for that matter) into a variety of stores. If a model could be started through which a podcast producer can put their podcast in multiple stores so that it's a paid show but available in Spotify, Apple Music, Google Play, etc., that would be a nice model indeed.
It's doubtful that Spotify would attempt such a model.