Problem with deterrence through the US alliance is, you never know what Trump thinks of the Islamic Republic of Japan when Japan really needs the help.
As someone currently in South East Asia, and having visited S Korea, Japan, etc before, I have no issues with Japan, N Korea and Taiwan getting nukes.
We know what happens if you have no nukes and a larger country wants what you have (Ukraine and Russia). And we know the UN is toothless when a belligerent nation is a perm member of the UN Sec Council (or a close friend of a perm member).
I can even understand Iran wanting a nuke now after the attack by US and Israel. Prior to this attack, I was leaning towards Iran not having a nuke. Now I can understand Iran wanting it at the least to make a belligerent nation think twice in the future. Did you know that Ayatollah Ali Khamenei of Iran (the guy who got killed by the US) declared the production and use of nuclear weapons haram (forbidden). The new guy (who happens to be the son) is probably wanting nukes cos of what happened to his dad.
In conclusion, I can understand many more nations wanting nukes cos of the state of the current world. Sort of Atomics owned by the Great Houses in Dune. Deterrence against the others.