Putin seems to like turning back the clock on that kind of thing. Just because it's irresponsible doesn't mean he won't do it. He's a rational actor in terms of his own grip on power and nothing else.
Again, Cuba's no longer a vassal state. I suspect they have little interest in hosting Russian nuclear missiles, and I suspect the US would be more than happy to help them resist a Russian effort to restore the Cold War situation.
And it cost them forgiveness of 90% of Cuba's debts. I doubt the other 10% will cover hosting nuclear missiles, especially if Cuba's at the point where they're interested in better relations with the US.
Imagine that the US didn't lift the embargo but Russia offered access to Russian cars (yes, they exist), cheap oil and whatever else Russia can provide. That would be very tempting for Cuba, I think.
