However, I don't see any actual arguments as to why anyone should gain a massive advantage with spending lots of $$$ in a game.
Ok. Remove all advantages for money. What exactly is your plan for keeping up with running costs? 'Cause the only way the game stays is if it can cover all its yearly expenditure. Which under the current payment model it succeeds at
Skins only? Would that be enough? Subscription? A lot of people would never consider staying under those conditions without a dedicated server for it. That could theoretically happen but who knows what sort of price it'd be asking. And if you're paying a subscription is it stripping off all paid content? Or is the paid stuff included? One time fee to buy the game? Would anyone actually consider that if they're not already buying? What price would we expect it to cost knowing how much diamonds sell for?
On paper I'd love to have a game with even footing. In practice you'd still have to find someone paying the bills because it's online and has servers. It's not running solely on my own machine.
If I'm not willing to sacrifice even the smallest amount of money for that (now, not just over my time playing. Because you're asking to change it into even footing) then it'd be unreasonable to expect those that are currently spending to get
nothing extra. As a business it's not going to be voluntary contributions, it's business transactions. Which yes, if players spend massive amounts of cash that's going to compound into a massive short-term advantage one way or another regardless of where we draw the line on what that advantage should be. The only way that doesn't happen is if it's purely cosmetic or on something with no real gameplay value or you require everyone playing together to pay the same amount as each other