If Russia nukes Ukraine and gets away with it, what do you think happens in the future?

Don't you think that by "choosing to let them" in advance and communicating it openly - you increase the likelihood they'll do it? LE: Right now, not "in the future".

And then do it again, for the next conquest? And then other dictators like Iran, North Korea and China do it as well?

Increasing the likelihood in the future is better than a 100% probability now.

Would you be willing to give away Alaska? Getting it back is part of russian nationalist discourse. Obviously not of the mainstream one, but only 10 years ago subjugation of Ukraine wasn't a part of mainstream discourse either.

Ukraine isn't part of the US. We aren't giving away anything because it was never ours to give.