Well, if it had a longer stroke, sure. Really no measurable difference in performance otherwise. Probably allow after market cranks eventually anyway if someone gets the right payola.
Aftermarket cranks area already permitted. They are often referred to as the "Eddie" or "Dixie" cranks that came from China.
The reason to not allow welded cranks is because of the exact reason you offered - it is too easy to offset grind the journal and take advantage of the extra stroke that is allowed within the rules. Also, coatings are not allowed. Interestingly, cryogenic treating of the crank WAS/is permitted when Briggs introduced the R2 with a Cryo crank. Unfortunately, the process isn't very valuable on mushy cast iron in my opinion, and those cranks from R2 engines were some of the quickest to wear out for whatever reason.
20 some years ago, I had a customer who worked with a high profile professional race team send me a crank that looked stock enough (but felt slick like glass.) The process reduced the parasitc drag and windage in the crankcase (enough that it was measurable) and that was before guys started using acceleration dynos. It was a real nice piece but I didn't feel comfortable using it in the stock classes. At the time, this treatment was not available to just everyone, and even if it were, mere mortals couldn't afford it. Now I see similar treatments and coatings available fairly inexpensively since more people have gotten into this market. The benefits of coatings on pistons has been around for years, but just in the last 10 or so, have we seen treatment process and micro dimpling & polishing to bearing shells, wrist pins, and crank & rod journals.
Karting is NOT F1, and shouldn't ever require this sort of technology (limited to few) to be competitive. We have enough problems with some imported parts in certain current engines being available to a limited number of builders already. Even if the advantage are slim to none, the perception is that they are getting the "picks of the litter" each time a container is dropped on our shores.
Thanks and God bless,