The costs of all of it needs to be economically feasible even if the technology is there, I don't think welding is only reason we have been limited in the size
Who pays for this dredging and these cranes? The localities. It's difficult to defray these costs with shipping fees. American President Lines pulled their headquarters from Oakland to Phoenix when Oakland did that. [2]
[1] https://en.wikipedia.org/wiki/CMA_CGM_Benjamin_Franklin
[2] https://www.prlog.org/10551489-apl-phoenix-headquarters-relo...
I am not saying the limit doesn't play a major role, I am saying if it's was the main bottleneck there would be a lot of r&d in solving it, solving it would certainly help, but the economics and other limitations mean solving welding won't mean a revolutionary change to ship sizes
Expensive would be OK, of course a bigger ship costs more to build than a small one, but you hope to make the investment back by carrying more cargo and additionally benefit from some economies of scale.
However, the biggest ships we have are at the limit of how thick we can join steel plates with current welding technologies. So there's some non-linearity here.