Doesn't that problem still exist though? Say you sponsor a project individually and it eventually goes unsupported. Would your sponsorship live on if you never manually cancelled it?
Using stars as a proxy for sponsorship, I think, is the wrong idea anyhow. Sponsorship should imply star, but not the inverse. I think what would be best is an easy way to sift through your starred projects and "upgrade" them to a sponsorship. Then once you've done that once, you can manage stars and sponsorships independently going forward.
They could factor in the repo's "pulse". More active projects get a bigger slice.
I could see having some kind of "tip amount" per project that gets taken from a pool would make a lot of sense, but not as stars.