Podcasting has also been called the slowest growing new medium in the history of new media. I can't imagine it's earning its keep, relative to those big initial payouts. Meanwhile, the shows that are brought into subscription-only world systematically fail to attract new listeners; it is hard to use podcast exclusives to grow market share. Especially the talking head variety, which exist in abundance outside the pay wall.
But I agree that podcastings growth has been very slow. I think that there are a lot of factors influencing that. The biggest in my opinion is that listening to a podcast is more intimate than any other media (including broadcast radio) and that factor both slows uptake and decreases churn. Not a marketer though so just making that up.
Will it pay off? Probably not. But is 25% a big share? Yes.
https://en.wikipedia.org/wiki/Apple_Music
https://en.wikipedia.org/wiki/Google_Play_Music
https://en.wikipedia.org/wiki/Spotify
The problem for Spotify is that they need a far bigger share of the market to be able to have negotiating power against the 3 record labels that they have to buy the music from. Apple/Alphabet/Amazon do not care as much because re-selling the music at cost or even as a loss leader is not a big deal to them.