And I guess you can't even theoretically reduce it past 1/2^256 anyway. Which is probably precision of target number anyway. Thus, smallest would be a single private key level of difficulty, and according to books that takes a thousand suns a thousand years lol
Discussion
That's what I know. And it is indeed a theoretical, even rhetorical, question. And, as far as I know, it's a one-to-one function which means if btc ever reaches that difficulty the network will halt forever, since block production will halt and difficulty adjustment requires a certain number of blocks to be mined.
Haha yeah, guess there's plenty of room to raise it left. It actually has a max difficulty and min difficulty change hardcoded I believe, something like 200-400% swing allowed I think? So a huge increase in hash would speed blocks up by max until proper diff reached.
Interestingly, I think anyway, is that there are less than 2^256 unique points on EC (close but not quite), so even the limit is more than enough