Upkeep does it job in keeping player gear somewhat low (and diverse), and that's all it's meant to do.
It only keeps the gear somewhat low for your new players. High level players have stated(in this thread) that it means nothing to them. Having 1 mil gold, makes upkeep points less. The only people you are impacting are the lower levels, probably only the first gens. Who are the key to making this game more popular.(maybe better?)
I don't see how all the high level players running around in plate is a problem? Have you tried playing your game all over as a level 1? I have, and I am level 20 and just played with a friend who is level 1. We went to a server to just 1v1 and he could hardly even do damage to me. By hardly, I mean he probably hit me 10+ times before I died. Not to mention he would hit me probably 10 more times along with that, and do zero damage. That's 20 hits to bring me down.
Now I understand he is level 1, but is that really the player you want to hinder anymore than that? Already new players are having to go up against people that are the equivalent to gods, you really should discourage them buy hitting them with a 5-600+ gold tab at the end of a round.
The fact that you don't have everyone wearing full plate now, has more to do with players making choices on who/how they play. If players migrate toward one set of items over and over, it's a balance issue 90% of the time. I'm not asking you to have a dedicated team to keeping shit balanced, I understand this is just a mod. Just throwing upkeep into a game like this for the reasoning of, "Oh, well this will give them X amount of more hours to get to the top before they get bored" is not the best reason.
If you are actively supporting this game,(which it seems you are) there are so many more ways about extending the life of the game. Instead of what you are now currently doing to it by killing it. By killing it I mean you are not increasing in a new/bigger playerbase.
I only say this, because I really like this game so far, and this mod seems to be the only one adding to it.