Even if the Boxing Ring didn't have that stage hazard, and wasn't banned, odds are that tournaments would have used the FD variant of the stage, since it would be the same exact stage but without the walk-out stairs and bouncy ropes. It would be one thing if it was a Battlefield-like stage (in terms of platform placement) that was banned, but Boxing Ring is already a mostly flat stage, so not much is lost becoming a FD variant.
Seriously though, Melee is not going to hurt Smash 4's sales. The Smash Bros fans that are so attached to Smash's competitive play as to not want to purchase this new one are, while quite vocal on the internet, far fewer than they appear. Speaking as a fan of the competitive scene, I am very hyped over Smash 4.
Project: M and Melee may better serve the competitive community, but they do not make me any less excited when I see that Midna and Ashley are Assist Trophies. I'm buying Smash 4 when it comes out, and I will have fun with it. When Project: M gets another big update, I will have fun with it. When Melee has another big tournament stream, I will watch it and have fun with it. I can do all of these things, and so can other fans of the competitive scene.
I am sorry if I'm just whining and venting, but I have seen FAR more criticism against the competitive community by casual Smash players than the other way around on this thread, and I am so tired of it. If these two subsets of the Smash fandom could co-exist, that would be fantastic, but the never-ending and incredibly pointless arguments between these fandoms are still going on everywhere, and I am so tired of it.
<sigh> Sorry for the rant.