OTOH, I think that it's still a bit wrong to say don't solve them now: if you have a problem that is potentially a business-ending disaster that you won't have time to fix before it destroys your business if it manifests, it doesn't matter if the conditions which would cause it to manifest are otherwise an improvement over your status quo conditions. It still needs fixed before that happens.
Otherwise, yeah, YAGNI applies.
On your second paragraph, 100% agreed. This is really just a useful prioritization heuristic, not a rule, and in some cases other considerations should and will override it.
It's mainly useful just to deliberately and routinely check that instinct to go deep on whatever you're working on when there's a lot of breadth to cover that's more important to get done first.
"Is this feature important, or a nice to have" is the question they want, not "is this problem nice to have?".
> nice problems to have relative to the status quo, so problems that are really only potential problems right now, but could become actual problems if good things happened
But that's not what this very common phrase means.
What they want to say is "nice to haves".
For many "nice problems to have" I really disagree.
A nice problem to have can be a real problem. It's nice you are in that situation but it's still a problem.
Your site offline because of demand is a nice problem to have. It's absolutely one to solve right now.