To dissect these one by one.
Fighting games in general are a niche markekt, something the FGC itself is well aware of, and why Fighting Games nowadays try to incorporate easier methods of play to try to rope in a more casual audience. Contrary to popular belief, casual players don't want to spend hours upon hours in the training mode to map out combos, they wanna mash buttons and see big damage, it's why a lot of games are adding in things like auto-combos specifically for that kind of market, and partially why Smash is such a big success. Sure, you can play at insane pro-gamer levels, or you can mash buttons and funny wahoo man punches.
I'm a bit late on this, but while I do agree I should note that a lot of casual players will inevitably run into those kinds of situations where they wonder "What the hell am I getting whupped by?". And that leads into another point - there'll always be a segment of the casual playerbase who looks to improve. Sure, probably not thousands of hours in Training mode, but it's not difficult to imagine that quite a few casual players want to get at least an edge on the others in the room if not follow those they see on Youtube. The clearest example I've seen of this in Smash is
: if you ever see a Luigi player, chances are pretty great the player will at least attempt the zero-to-death shenaningas. In one on one battles, at least.
(As an aside, I've noticed via Spectate Mode that casual one on one games tend to have its players at least lean a bit more towards competitive / tryhard - items show up much less there for instance - while it's mainly FFAs / doubles that host the casual funny wahoo man punches. Echoing the old "For Fun / For Glory" divide.)
Now, the "a segment of the casual population looking to improve (some of which become competitive or even pros in the process) while another just wants to see funny wahoo punches" is a general thing regardless of game, but it gets to a thorny question: How does one make sure that emerging tactics like lets say Luigi zero-to-death or projectile spammers do not become too much for a playerbase and it become alienating? It's extremely difficult since such tactics do not neccessarily show themselves in all skill levels: more experienced players will have much less of a hassle navigating projectile spam and countering those players' usually lacking adaptation skill, for instance. As a contrast a lot of casual Luigi players don't know how to execute the shenaningas (doesn't stop a lot of them from trying though).
I wouldn't be surprised if Sakurai and co. nerfed projectiles' shield damage as a whole (patch 3.0, Joker's) partly because a lot of people were having trouble with projectile spam (ditto Min Min's shield damage later on, which to me is telling). They also nerfed Luigi's D-Air so it wouldn't spike as easily (and thus increase the zero-to-death's difficulty) around the same time,
Now, communicating counters to such tactics to players - like using ambigous movement to avoid projectile spam - can be rather tricky, and it doesn't help that Training Mode is sadly a bit too bare bones. You can't set a CPU to Side B without mods, so training vs. say Samus' Missiles, PK Fire or Blazing End is next-to-impossible in Training Mode. That is something I'd like to see them fix next game, alongside giving a more detailed command list. People won't need to spend thousands of hours in Training Mode labbing out comboes, no, but something that like that would help in situations they might encounter rather often.
Also, this video not only reinforces your main point, but also shows how I'd like Smash 6's Training Mode. Please integrate it with the tips system, allow people to practice specific scenarios instantly, and allow players to set the kind of goals they want to achieve:
Ultimate is balanced, has a huge cast, there's not one character who's completely dominant (Aegis comes closest so far), there's just one player who's very dominant and he's playing a character that some people don't even think is High Tier, namely Byleth...
Just take a moment to contemplate that, a Byleth player is the king pin of Smash Ultimate.
That's reasonably well balanced.
My only issues with Ultimate's balancing is that it can be same-y amongst the cast and platform interactions can be really limited, but that goes into the thorny question of depth.
Still, it's saying something where
the currently-releasing top 100 players list includes not just the said Byleth player (and yeah, I'm 100 % sure he'll make it in when the last part, 25th to 1st, releases today), but also a distinct variety of characters, such as an Isabelle main.
As for Melee's engine being superior that's a misleading argument. The reason why advanced techniques even happen in the game is because of the holes in the engine that weren't fleshed out beforehand leading to essentially glitched gameplay. That is not "superior."
AsumSaus - who loves Melee to bits - has coined the entire process as (to paraphrase) "Hey, lets make something beautiful out of this broken disaster of a game" and it's pretty damn apt. Yes, depth can certainly be really alienating if people start to lean onto it a lot (just see the
TV Tropes article on the Casual Competitive conflict), but the ability to experiment with games can be very good for a game's longevity. It's a tough balancing act, and one that is not always that clear cut.
Still though, the fact that it's still being talked about 20 years later and it has influenced the development of subsuquent Smash entries says a lot about Melee's legacy.
FWIW I think it's worth noting that Melee is pretty balanced
amongst the viable characters - Samus, for example, has an even MU with Fox / Falco despite her being like, 11th - and most of the other characters can and have pulled off big upsets. Sure, it's unbalanced overall (poor Mr. Game and Watch - several of his aerials were initially mapped as B moves), but it's not bad for a game made in 2001 and just 13 months.