It’s indeed sad that some publishers hide their content in a closed ecosystem. Every time that happens with a podcast that you like, please do let them know this shouldn’t be the way to go.
A similar request as yours was made on GitHub (where technical development happens). My reply there:
this is a project by an individual, and we don’t know what might happen with the service that converts Spotify shows to RSS feeds. Individuals can use the service and add these RSS links to AntennaPod, but I’m hesitant to offer this in the search in AntennaPod.
Our users would still rely on an external site (spotifeed), but without realising. If Spotifeed at some point goes down (gets stopped, killed by Spotify, …) AntennaPod will get complaints by users whose podcasts suddenly stop working.
Including a third party service that recreates feeds for other’s content is (IMHO) too risky to implement.