If MLG is running again but for Brawl, this will still help Melee and Smash in general. How many Melee players are being converted because of this? Not likely too many. How many newcomers are going to go to Brawl instead of Melee now? Likely quite a bit. How many players will be converted from Brawl to Melee or become aware of Melee through MLG. A lot.
This will help Melee, and for the Brawl haters, perhaps slow down interest once the brokenness becomes more apparent on a larger scale.
If the Melee community continues to have colossal tournaments like the RoM series, Genesis, Pound series, APEX and others, Melee will continue doing what it has been in the last few years. And it's not as if the game as stopped accelerating; before Summer 09, nobody would have fathomed a Peach destroying USA's top 3, we finally got to see an end to Mango's long chain of tournament wins and we've seen new levels of Jigglypuff, Link, Pikachu, Ganondorf and Sheik (among others). Melee is fine, and MLG won't do anything negative to Melee.
Melee overall just needs to continue growing and expanding. With APEX coming up, a large chunk of Europe is going, Canada, players from all over USA and many, many others. Armada did a really great thing opening up what's still out there in Melee, and Amsah supported that further. There are still many other players at incredible levels that have never competed against USA, Europe and other places such as Calle W, Javi, Twin, Dark, Dieslow (I think), and others.
Also as a Smasher for all the games, I think MLG could bring some interest in what Brawl could still become. Granted, most Melee players lock themselves in a basement at the words of Brawl, but there are many Melee/Brawl players and I think MLG could make Brawl somewhat more interesting. It's exciting that there are different people winning (ADHD, Ally, M2K), rather than the same guy everytime (Mango).
And if not and Brawl crashes and burns, then maybe Nintendo will actually put some heavier testing into the next Smash game that all three Smash communities can enjoy.
Why I like Brawl: Almost purely mindgames to win, different people take the top spot, larger selection of characters (more variety), you can learn a larger variety of characters at a high level.
Why I dislike Brawl: Mostly MKs, slower and not as combo-y as Melee, lots of abusable tactics.