See, here's the funny thing. You're actually wrong. I could load up my low level party with a bunch of rare items (Potions of Supreme Healing, one each) and it would make zero difference to the party power level, OR, I could give one rare item to each PC and MASSIVELY increase their power level - as in nearly double their DPR.
All using the same level of magic items. This is worse than no system at all because the rarity scale in no way actually addresses the power level of the items.
I agree that any system that does come out will not be perfect. Absolutely not. But, we can have a system out there, and then that system can be tweaked repeatedly from feedback from the fandom. That's what Unearthed Arcana feedback is for after all. It's not like WotC doesn't have access to millions and millions of played character sheets that they could then use to develop a relative power list for magic items. If Item X appears on 60% of character sheets of a given class, then that item is obviously very powerful/useful and should be valued as such, or perhaps even nerfed in order to bring it back in line with other items of the same level.
I mean, right now, a Cloak of Protection is an uncommon item. A Ring of Protection is rare. They do EXACTLY the same thing. +1 to AC and saving throws. These items, other than one is a cloak and the other a ring, are identical. How is the rarity value even remotely helpful here? And, heck, because of this, the Cloak is FAR more easy to come by using the Xanathar downtime rules. And about 1/10th the cost.
That's where transparency comes in. Why is the cloak so much more readily available than the ring? What's the reasoning here?