Let us say it comes out, I happen to like it (I like everything so I probably will), so I go decide to try and create a group together to hold weekly tournaments. What are the chances of no one showing up because they only play Melee, and me just having to settle with playing with my close friends, and people online?
Depends, what does the final version of the game look like? Some melee fans are adverse to new smash games simply because of Sakurai's attitude toward them and the major disappointment brawl was for them. What they found fun is no longer being supported.
The way I think of it is like this. You have some kind of obscure thing you can do well in and have a lot of fun/enjoy it and maybe earn a bit of money off of. But your parent who you respected just doesn't support you despite your success and actively says what you are doing is "not right". And even far down the line they still are disapproving and refuse to understand or change despite how far you've gone with your own effort and money.
Sakurai constantly says he has to appeal to a demographic that doesn't need appealing to. These people will buy the game regardless of the mechanics and will very likely will buy the game almost regardless of what is put in there.
And then there is the deep mechanics issue/balance issue. Mostly being able to act out of certain actions like dashing or landing on the ground during an air move after minimal lag.
Honestly it is totally fine to be skeptical about the game after brawl. Heck, I'm super skeptical of a paper mario game ever being good again after super paper mario and sticker star. I refuse to spend my money on a new game despite loving paper mario if the quality or what I expect out of the game is not great. And I think your post has heavy anti-melee undertones that really I do not care for. You have nothing to worry about if the game provides enough substance for these players. And if it doesn't that is not their fault nor is it yours.