I sort of have the opposite situation. I refuse to play with proxies when I can manage it (sometimes I will use them when the cards are in the mail or when I am testing cards) but for the most part, I don't play with proxies.
My decks still end up having similar cards to one another, though I make a conscious decision to try to keep the overall "feel" of the decks distinct. So, all my blue decks play Cyclonic Rift and Mana Drain for example. And, again, none of these are proxies so it isn't like allowing for proxies is pushing things in my decks to be degenerate; I have the cards to build whatever I want and I want those two cards because they are good (Rift especially being a sort of "get out of jail" type card).
So, with that being said about what I do with proxies, I have done my best to really encourage proxies in my group. There is one player who proxies his entire deck which I am fine with. And others play generally lower powered stuff because they don't want to use proxies, which is also fine. But I keep trying to push for the "proxies are ok" mentality because it shouldn't lead anyone to any particular playstyle.
Maybe things are a little homogenous as players start out, and maybe they don't evolve from there but that is a playgroup issue and seems like it is a weird thing to accept and/or advocate. That is, it seems like your point is that players would dive more into homogeny and optimization except they are simply priced out of doing so. Which....I guess I don't see the point. Plenty of cheap cards offer homogeny; plenty of powerful cards are cheap due to reprints. To suggest that you don't want to see too many Scroll Racks simply because it is expensive while you are fine with Kinnan combo simply because it is cheap (enough pieces are anyway) seems sort of backwards.
I have a feeling that if you really cracked down on proxies, at least from the "too powerful" or "optimized" category, and then someone like me showed up to your group and played every blue deck I had which includes all the stuff your group has said was not ok to proxy, I imagine it would feel sort of unfair. I mean, what is really the difference between proxying a Rift for every blue deck and simply buying it from a play perspective? You force someone to spend money, sure, but your games are the exact same as they would be with proxies.
If there is an issue with power or whatever then talk to your group. Encourage diversity but don't do it through some sort of gatekeeping based entirely on personal budget or finances.
In short, I think there is plenty of degeneracy that can occur without proxies; that is a player issue, not a budget issue.