Except that you don't need a hacked wii for BBrawl. Research, bro.
Oops, shows how much I know about hacked versions... which in a way kind of supports my point.
The problem doesn't lie in the good players. If BBrawl became the standard, which I doubt it ever will, then you know the good players would switch over and keep playing.
However, despite what many may think, they aren't the ones that matter. The people who fill the pot, people like myself who aren't good enough to make money for whatever reason (Not competitive and attend because friends convinced them to, those who attend for social purposes, people who play casually and hear about the tournament through venue advertising, those who don't have or take the time to improve let alone learn new game mechanics, etc.), as well as anyone who's character suddenly gets worse, are the ones who are more likely to stick with the original version. Sadly, these are the people who supply the money the top players then take, and without the money, all the top players migrate to the game where they can win money.
Now the main difference with DLC is that DLC comes to you. Take for MVC3 for example, since you bring it up. I play it, and I've attended a tournament for it, but I don't actually have the time, and probably won't ever with the schedule I have, to learn to get good at the game. Hell, I've barely even looked over Shuryuken for anything other than where tournies/MVC gatherings happen. If they had developed and distributed Jill and Shuma-gorath on the forums, I wouldn't know about them. However, if every time I get on the game it reminds me that hey, there's this update out there that it'll download for me in about 5 minutes if I want to (The shadow battle thing atm, but will probably do the same for the characters when they hit), then there's a much larger chance I'm going to get it.
TL:DR version: The pot-fodder player is more likely to accept change if it comes to them than if they have to go out and get it. The reason DLC works is because the people who made it are trying really hard to get you to buy it, one of those ways being that you are excluded from the group of people who do have it (Black Ops for example won't let anyone in a party play first strike maps if any single player in said party hasn't downloaded it, which leads to peer pressure to get the DLC and join the fun everyone else is having.). BBrawl pretty much says "I'll leave this here. Come get it if you want it."