I don't see it yet, please help me out. Maybe it helps if you can find a specific example.
Can you describe a scenario how an asshole could game that system?
Generally I think MTG is probably the most capitalism-ruined game. It annoyed me much when starting to play as a teenager. Whenever a friend upgraded their deck, others were kind of forced to spend money as well. Because the rich guy had access to all the powerful cards (= relatively low mana cost for their effect / strong effect for their CMC). Isn't that exactly what a balancing approach would alleviate? Everyone has access to all cards, and all cards receive a CMC which matches how much players value it.
Excellent point. Yeah, some things exist which are already kind of broken and could be exploited further. Maybe more generally, MTG uses CMC to balance cards, but also many other aspects (comes into play tapped, draw a card/gain life on enter, ...). So only changing mana cost affects the balancing of different cards differently.
Yes, Llanowar Elves. A silly result could be that people use equivalents (like Elvish Mystic) while the "original" is too expensive. But probably, all ramp cards would probably become more expensive as long as they do the job. Which makes ramp cards kind of pointless, as you point out.
Right, two good objections and a funny video. I like it, thanks.