Is there a feature you want out of the paid subscription, or do you just want to support them? The subscription goes towards enabling features which have an ongoing cost (sync of files and listening state across devices so you can listen on desktop where you left off on mobile). I don't think they want to offer a one time fee for what is an indefinite period of cost.

AFAIK most features other than that are still fully supported in the free version of the app without ads though. So if you don't want any of those features, they're probably better supported by you recommending them to others than giving them a few dollars.