You can 100% continue to purchase content and add to your library. Just because the apps have split (as they already have on Apple’s own platforms), does not mean you can not download purchased content or continue to purchase new content for download.
Buy music from the iTunes Store
If you don’t subscribe to Apple Music and you want to add your favorite songs or albums, you can buy and download music from the iTunes Store. And if you previously purchased items from the iTunes Store, you can download them to your authorized computers and devices (up to 10 total).
That’s only for music you already bought just like Google did with Youtube music when they killed Google Play Music
You can 100% continue to purchase content and add to your library. Just because the apps have split (as they already have on Apple’s own platforms), does not mean you can not download purchased content or continue to purchase new content for download.
https://support.apple.com/guide/music-windows/build-your-music-library-mus0f01bdce4/1.4/windows/10