and that is why standard sizes exist. The solution then, is to get every one on standardized sizing so the parts work.

The problems are that now somebody needs to have and maintain a printer. (I presume FDM, but resin or something would be more useful.) even then, the production is limited, and you're still having to source filament or whatever, as well as the parts necessary to maintain it. These parts are all more complicated than PVC pipe fittings are. (and depending on where we're talking, may not even be available for a variety of reasons.)

and when the part fails (as all things eventually do,) you're going to be right back where you started. You might be able to get another custom part printed. if the guy happens to still be around and availible.

Granted, the people running the study or setting up the open source fittings probably don't have the money to encourage people to replace their plumbing, but that's the solution that won't see us right back at the problem in a few years. or less.