I guess I still don't understand this. What is the window where something is similar enough to be a "Dominion spin-off" instead of just another game, but not so similar that it could have been a Dominion expansion? I don't get the sense that you feel it, but there should be an enormous push to make anything sufficiently Dominion-like into a Dominion expansion so that you can combine it with everything else. It's really, really nice to be able to combine it both from a rules-learning perspective and for the sheer joy of seeing the old stuff with the new stuff.
At this point I feel the opposite - that if an expansion could work as a spin-off, that's a better direction for it. It's fun to combine all the cards, but you already have so much of that. A new Dominion expansion has a nice minimum built-in audience, but also a maximum one.
My idea of a spin-off is not to copy Dominion in its entirety and then add some mechanics, like lots of people have told themselves was good enough to qualify as a new game. My imaginary spin-offs are much more different from Dominion. Kingdom Builder was, you play on the board to get cards, you play cards to play on the board. It could not have been a Dominion expansion, any which way. Temporum I don't want to go into but it couldn't have been a Dominion expansion either.
Since obviously you're not going to get into specifics of what a Dominion spin-off might someday look like, perhaps you could just say, why do you feel the push to specifically make "Dominion spin-offs" rather than Dominion expansions and also unrelated games?
I would love to make more unrelated games too of course.
A Dominion spin-off gets to be a different game in whatever great ways, just like an unrelated game. It gets to have new simple cards, that do whatever stuff you can do in this game; it gets to have Dominion-like cards that don't have to worry about not being strictly better/worse than any of Dominion's cards. It has a bigger minimum audience than a random new game, but a bigger maximum audience than a Dominion expansion.