I agree completely with the OP, his proposal also minimizes community split which is extremely important. It's also dumb throwing away a good game in place of a bad-okay game with maybe goodish potential.
There are four cases here which matter, y/n melee stays main event, and y/n brawl becomes a better competitive game than melee
You can think of this in a cost-benefit analysis:
1. If melee stays main event and brawl stays bad, we've lost time to do other things, BUT we've given a possibly good game a fair chance.
2. If melee stays main event and brawl gets good, we've wasted time playing melee when brawl's meta could have been advanced instead, BUT we covered our ***** in case brawl was terribad, AND we spent less total time playing brawl with an underdeveloped metagame.
3. Brawl becomes main event, and brawl becomes a better game than melee. We have a strong meta, fast and possibly online tournaments for money. BUT we took a huge risk on an initially terribad game, AND we spent a lot of time playing a teribad game until it became a amazgood game.
4. Brawl becomes main event, and brawl continues to be teribad, doesn't matter if it becomes teraworse or terabetter, it's still terabad. SO we spent all this time playing a really bad game, didn't enjoy most of it, and it was just a massive time waste. In that amount of time, we could have advanced smash to the point where double's tiers matter.
Here's how community fragmentation goes in each of those scenarios:
1. Very little fragmentation. Almost all meleers will switch over, the ones that don't are just odd.
2. Some fragmentation, the nintenboy fans will still be playing brawl, and trying to play it competitively but all the pros go back to melee.
3. Initially significant fragmention because brawl so far seems like a pretty bad game. Once it is shown that brawl is more competitive most meleers will switch over. Although in the long run the fragmentation wouldn't be too bad, fragmentation in short term would be terrible, AND that's really when fragmentation matters, in the RIGHT NOW as new people are joining, communities and connections are being formed.
4. Extreme fragmentation. The melee and brawl camps become more polarized because brawl is not improving and melee is. Most brawl pros switch over to melee, all the fanboys stick with brawl indefinetely until ssb4 is released.
There's already extreme controversy over brawl vs. melee and it's rumored that m2k has already given up on brawl, anything that better unites the communities is a good idea.