Technically, bitcoin's difficulty could hit nearly 0 (not quite 0, afaik). All'd we have to do is remove ALL miners from the entire network, and only solve one block every few weeks. Eventually, after 15+ years, difficulty would readjust and be close enough to 0.

Curious here, instead of resetting the difficulty to 0, why even have a difficulty? Just make it the same hashing algorithm, and each hash is a reward. Simple as that. Then again, blocks would be found at a crazy rate unless the hashing method would be super intensive (32k rounds of sha256, another 64k rounds of bcrypt anyone? Then the results of 16k sha256 rounds added up together to create a single hash).