Ideally no one, and if the cost / expertise is so niche that only a handful of sophisticated actors could possibly actually do it, then in fact (by way of enforceable treaty) no one.
We're talking about making war slightly more expensive for yourself to preserve the things that matter, which is a trade-off that we make all the time. Even in war you don't have to race for the bottom for every marginal fraction-of-a-percent edge. We've managed to e.g. ban antipersonnel landmines, this is an extremely similar case.
> How would you enforce it after you get nuked?
And yet we've somehow managed to avoid getting into nuclear wars.