Your train of logic is incredibly flawed.
Brawl was the biggest smash game until we learned there was a sequel coming out.
There were still Brawl majors happening up until Smash 4 was right on the verge of being released.
Brawl 'died' because it had a successor coming out, which is the same way 99% of other games 'die' competitively.
Those who are blinkered in competitive gaming fail to realise that Melee damn near one of the only games to not die after its succeeded.
Also, while I appreciate that some people are suggesting ways of making 3-stock the norm via weaning the community onto it. Having tournaments be half 2-stock and half 3-stock isn't really going to work. It'll only serve to confuse people more. If you wanna do it, you're gonna have to go headfirst and change it completely
Flawed? Let's see then.
Brawl is now dead, it didn't survive Smash 4 while on the other hand, MELEE IS BIGGER THAN EVER and it's still growing....
Is it bigger than ever because the players changed? Because we saw a huge peak of new players appearing up and down on the top eights and stuff? No, we still see the old folks most of the time, it's bigger than ever because THE AUDIENCE IS BIGGER THAN EVER.... those are directly proportional, it's not a coincidence.
And even if all the old Brawl players migrated to Smash 4, do you think Brawl would have died if everybody kept watching Brawl? Hell no, it would still be alive and health. Meanwhile, if all the old Brawl players stayed there and Smash 4 saw new stars rising and showing they tech those instead of the migration it actually happened, do you think Brawl would have survived? No, because Smash 4 would be much more exciting to watch, even with ZeRo, Nairo and such still on Brawl.
And on that matter, it's not the players who will decide whether PM lives or dies, it is the audience who will decide if it's still worth watching it, and thus, the fate of the mod.
Nobody makes sports for no one to watch, the thing you're thinking of is called friendly match, like those on the couch, at home.