I often would like to tip someone without establishing an ongoing relationship with their site/work - and I'd think that they'd rather get something instead of nothing from me.

While it may be more sustainable, I have to say that personally I strongly associate the subscription model with all sorts of late-capitalist scams, from gym memberships to Juicero. Which isn't to say that I am opposed to supporting people creating cool, ongoing work, but that's the exception, not the rule, to that model.