Stages were removed to increase competitiveness and advance the metagame among characters. Not to improve how well people could abuse stages.
The metagame has advanced the last two years with this ruleset, not with the old MLG one.
These things are not inherently true. That people can believe this blindly in a game theory discussion is frightening to me. Look, there's no official definition anywhere, but I would hope that we could agree that the only way a metagame can "advance" is under two circumstances:
1. Usually what people mean is that the metagame size is growing, that is, that the game is becoming a deeper, more interesting experience. This suggests that you are increasing the depth, testing a wider variety of skills, and increasing the volume of relevant knowledge. The meta regresses when the size decreases, as broken and dominant strategies emerge.
2. An alternative or complementary interpretation is treating the metagame like a timeline. At a given point on the timeline, unexplored options in the future allow options in the present to succeed that otherwise would not succeed. In this case, the meta can actually regress if there are not enough players pushing it forward, but it is rare and hard to lose knowledge once it is gained.
So Removing Stages - does it "advance" the metagame under either definition? A hearty No - at least to any definite conclusion. In the case of 2, removing stages could function as either a regression or an advancement. If the stages themselves became broken+dominant strategies, then removing them would be a benefit. Pipes was a casualty of this that everyone agreed on - people had started to counter people there with Fox/Falco even if they didn't play the characters and scoring wins. However, evidence of this occurring on many other stages outside the kinds of disparities you already see on "neutral" stages is inadequate.
Under case 1, removing stages only advances the metagame if you're increasing the depth, testing a wider variety of skills, and increasing the volume of relevant knowledge. Now, analyzing the depth tree of a game like Melee is near impossible to begin with. With that said, it's extremely hard to make the case that removing stage variety does ANY of those three things (save for the example above). On the contrary, it's narrowing the scope of skills. People on that side of the argument have repeatedly echoed that straight up - "Those aren't skills I think are important or that I think matter."
That's fine, but you aren't ADVANCING THE METAGAME BY REMOVING THEM. That's trying to have your cake and eat it too. You are purposely regressing the metagame to only that which you think is valuable. If someone actually wants to make an argument from character dominance, that would be progress, but I've never seen anyone do that.
The lesson of this story: It's not the people on the pro-stage side that are making decisions on fun and personal value - it's the anti-stage people who do this the most.
EDIT: Also, not gonna lie, this post deserves it's own topic. But I won't do that.