I am of the same opinion as he. It is not ideal that Japan or SK get nuclear weapons but depending on the situation and power dynamic, it may not make sense for China to use military power to stop it. If it happens, I also believe it will be very manageable for China and it will not change the power dynamic or calculus at all. Japan is a country that is so frail that Chinese economic hostilities can put them into recession, much less trade bans. So what do we have to fear from them?
In other words, we can game it out. Let's pretend you're Japan and you've acquired nuclear weapons. I'm China. Your economy is languid; my everything is booming. The US will not dare to fight kinetically after realizing its limits in Iran and American power is receeding in Asia to a rising China. What will you do to make trouble for me?
I have to agree with this. If I am playing as Japan and my economy is being overshadowed every year and already lost conventionally, there is no reason to throw in the lot with US/Taiwan to nuke China. Is a game losing move that has no benefits at all except benefiting US by helping them firing nukes and taking a part of the nuclear retaliation.
I'm not convinced - I believe that the more hopeless the economic situation is for Japan, the more likely it is for Japan to pursue rash actions. The most dangerous type of person is one who has nothing to lose. Once you get to the point of even considering nuking another nuclear power - are you really going to be thinking about economic repercussions? "Yes, I am aware that they have 1500 nukes available for a retaliatory strike - but at the moment I am more concerned about them retaliating with a trade embargo!"
Why is there "nothing to lose"? Despite being economically behind, countries are able to move on and continue to exist. It is not a zero-sum game if China is winning that means me playing Japan is game over. There's nothing to gain by nuking China but everything to gain to make a new partnership with China over time.
If economy is shit, it would be very unpopular to join a foreigner's war.
If I am china and sees Japan declining further and further especially trying to afford a nuclear delivery and deterrence with chinese trade sanctions. I agree with manq that not attacking is the smarter move. China should never have ambition to take over japan or end it's existence to give them the reason to use the nuclear weapons.
Japanese election comes and goes, the next candidate could be anti-nuclear again and ask for sanction relief in exchange for denuclearization.
If history has shown us anything, preemptive attack to nip a potential enemy at the bud is not worth it. Germany did that in WW1 against Russia with France financial backing to modernize.
Japan did that to US in Pearl Harbour.
Russia is doing to Ukraine to prevent them to join NATO
US is doing it to Iran right now.