Thank you for going constructive - it's all too rare.
I absolutely agree 3E was far from perfect, and made several missteps.
But detractors all simply wash their hands, saying "that proves it's impossible". And WotC likes this idea (which is nothing less than pure FUD), because they (not so) secretly
don't want to do the hard work.
They much rather continue on the path to destroying the core gold hunt that is D&D - just look at Xanathar and the excrecable "treasure point" system. It's abominable to think this would ever be adopted wider than just the AL.
A much more constructive and positive way forward is to tackle the design issues right on. You mentioning the shopping menu nature of it all. We all agree that's probably not what was intended, but we also acknowledge that's how it ended up.
This situation demands that the system is complemented by a rule detailing availability, and not merely as a vaguely optional/variant rule either, but as something that's built into the core of the system.
And item creation probably needs the "formula" or "ingredient" solution, again as a core non-optional system, where you can't create diddly squat unless you have the items required (thus allowing the DM to veto specific items). I prefer ingredients over formulas, since with a formula, once you've opened the gates to a certain item, it can be manufactured over and over again. For some items, that's fine. For others, not so much. With ingredients, the DM can control not just which items that are possible to make but also how many.
What is absolutely necessary is much more robust DM assistance. What items are appropriate to hand out to your heroes?
If the rules mandates formulas and ingredients, it should also provide default recommendations. Something 3E never did until it was way too late. The devs did learn loads about the way the system worked (as opposed to how it was intended to work). And I'm sure Pathfinder ("version 3.75") have amassed even more knowledge.
So creating a second (third? seventh?) version of d20 item pricing and creation should be entirely possible.
You touch upon more points, but I'll stop here.
My point is none of this is easy. Anyone claiming this could be whipped up by just some fan on DMsG is just talking through their ass.
In fact, this is hard. Very hard to calibrate just right. And time-consuming since attention to detail is paramount. The ones that ought do it is the core dev team of WotC.
And even the highest seller on DmsG reaches just a fraction of the audience of the worst-selling official 5E publication. To gain system-wide adoption (while still entirely opt-in), it absolutely must be in an official hardback like Xanathar.
