Will Apple ever allow stand alone Spotify streaming on the iWatch?

That is Spotify's decision.

Pandora can stream to the Apple Watch, so there is no reason to stop Spotify, except a Spotify decision

https://appleinsider.com/articles/20/02/14/pandora-watch-app-no-longer-requires-the-use-of-an-iphone-to-stream-music

If Pandora can do it, then so can Spotify.

Contact Spotify and ask them when they will support the Apple Watch.

It's not down to apple read here;

https://www.wareable.com/apple/how-to-use-spotify-on-apple-watch-6609