Like I said, it's been like 14 years. If someone was going to master them and have them on command 100% of the time, it would have happened by now.
Amsa Yoshi is pretty much an instant counterexample. Yoshi's DJCs have been in the game forever, but this year was the first time anyone really made Yoshi look totally viable. People are now drastically rethinking some matchup charts in this 14-year old game.
Alternatively, check out, say, Apex 2012 vids. That's still 12 years in, and yet the level of play is still noticeably better now.
I've watched approximately 3 competitive Brawl sets. It was a lot of turnip throwing, Meta-Knight poking people, and then a lot of running away until the timer ran out. It was horrendously boring. I'm sure that this isn't the standard(at least I hope not) but taking out potential execution flubs only helps to lead to a less punish-heavy game. And I'm not saying that there is isn't room for excitement without execution, just that there is inherently less because of one less evident force acting on the outcome of the game.
I will agree that I find PM more interesting to watch than Brawl, but that's just because there's more viable characters and play styles. The biggest issue with Brawl is that the lack of combos means the balance is shifted pretty heavily towards defense, resulting in a very methodical, slower-paced game. PM restores the combos, so even without the execution barrier it's a much more explosive game.
Delayed gratification. If something is easy to do, it's not satisfying to accomplish. It's a very visceral feeling when you master certain ATs and it feels good. I have a friend who was excited about learning to short hop the other day. Taking that away detracts a lot from the learning experience in my opinion.
Beating someone at the mental game is also super satisfying. When you know someone is going to make a certain play, then they do and you counter it perfectly and win, you feel like a total genius. IMO, it's actually much more satisfying to beat someone this way then because they made a mistake.
Of course, when you lose in a similar fashion, you feel like a total idiot, because you did something dumb and they saw it coming a mile away. But I feel that makes you want to get right back in there in a way that losing because your inputs weren't quite on point does not.
This is really one of those places where more people need to try Divekick, because every Divekick player knows exactly what I'm talking about here. I've played a ton of fighting games, and Divekick is easily the most satisfying one. Making a certain opening play in the first four rounds, then doing the reverse on the 5th round where they get flustered and try to anticipate it? I defy you to find better satisfaction.
I believe that Melee players who play PM as a side would potentially quit.
It's possible, but people say this about every change that PM makes (we can't change Melee character X, all the Melee players will quit!) and yet the game only seems to be getting more popular all the time. I'm sure there are a few players who basically just play PM because the Melee tournament is over and there's nothing better to do, but there's also a lot of people who genuinely like the game for the new things it brings to the table.
I don't think it would have much of an effect on the global level of play.
If not than people's fears that it would make their hard work trivial are unfounded. However, I do think it would result in a noticeable boost in mid and high level play. Remember, wavedashing isn't the only affected technique, Dacus, jump cancels, and OOS will also become more consistent in the lower levels.