Multi-polarity isn't anti-fragility. Multi-polarity is bad with no redeeming qualities, you get more war and more nuclear proliferation.
How much of the last 40 years have we been without a major war or "police action"? I don't think the world was as stable as you thought it was.
More self-reliant like North Korea?
The American order that stood for decades was anti-fragile. Everyone outsourced their defense to some reasonable degree, so no one was strong enough to pose an existential threat to anyone else, so we could all afford to be off guard.
Now every actor needs to have the strength to deter all others, which means everyone gets stronger, which means everyone needs even more strength to deter others, or deterrence simply breaks down. We had a multi-polar world in the past, it went terribly and led to massive wars. Now we are returning to that system but with nukes.
England, France, Israel... They all had nukes the entire time. What you are claiming sounds good but does not align with reality. You are overstating both the degree to which things have changed as well as overstating the degree to which the alliances have changed (i.e. Europe is still Europe. Poland won't be arming itself to protect against Germany or France, etc).
Of course Europe always had some ability to defend itself, but I think it's clear that some of that ability was outsourced to the US(with reciprocal benefits for the US, but still). Yes, this introduces some redundancy into the Western sphere, but that's a good thing.