Objectively Brawl didn't lack competitive success though. That's where none of these arguments even make sense. There have been tons of Brawl tournaments, and they were very successful. There were consistent winners and losers, there was a vibrant metagame in which no player ever reached the limit of the game's depth, and the participants were satisfied with the games. There were some ruleset controversies, and certain types of play that saw success were unpopular. Neither of these problems was substantial enough to really get in the way of the tournament level play though; it carried on regardless. It's not that this is an impossibly low standard either; plenty of games fall short or fizzle out (remember everything that happened with SFxT?), and Brawl did not. I'm not sure what rubric you're using to judge competitive success, but it seems impossible for any objective standard to call Brawl a failure.
This is where people get upset over the divisions. The Brawl community was just doing their own thing and being successful at it, and then when people come along and loudly insist that Brawl is terrible and that the game is a competitive failure, it's a denial of what those people have been doing. No one can say that you have to like Brawl, but it's, for lack of a better term, just plain rude to deny the efforts and successes of its community. Us dedicated Brawl players who are so loudly insisting we need to avoid doing this again with smash 4 really mean this. You don't have to like any smash game, and if you admit that it's just not your cup of tea in your criticisms and limit said criticisms to the proper time and place, no one will be upset. When you claim things like the game is uncompetitive when a lot of people are in fact successfully competing at it, you're not just trashing the game but also the people playing it, denying what they're doing, and it casts a malaise over the broader smash community to have that kind of internal strife. You can see that the malaise is so bad that you really have to badger us before we even defend Brawl anymore despite being a game we both love and have sunk thousands of competitive hours each into. For my part, I don't even care if you hate Brawl forever and think all kinds of crazy things about what we were doing at these Brawl tournaments for years. I just want smash 4 to be spared the same fate, and the only way I know to explain how that can be avoided is to explain what happened with Brawl. We don't all have to like the same games, but we do all need to support what everyone else is doing.