Seeing as Ice Climbers are being brought up a little bit, as a Brawl lover, I've dealt with "all the ****" and still continued to love and enjoy the game, but optimal Ice Climbers came along (pioneered by NAKAT) and for me, and many others, it killed the game. Ice Climbers' natural counter play was gutted in Brawl from Melee, with so many things given to them to make them more consistent, it resulted in a completely binary strategy from them that no opposing character could do anything about.
In short, dash forward with Ice Climbers
either: Dash grab, which has a massive upwards above their head disjoint (for no reason at all), allowing them to grab most character's aerials above their heads.
back roll. Reset the situation
You could not afford to attempt to read the back roll for the punish although I do try, if i'm 100% that's what's going to happen, but if I guess wrong I get grabbed and die, so for most players, they ... do not take that opportunity, because it won't work out for them unless they can guarantee hitting both climbers.
The game mechanics made this binary situation infallible. Natural desynching ensured that there was no way to hit both ICs at the same time in the dash forward, and if you hit Nana without hitting Popo while Popo back rolls, you're actually just set yourself up for getting dash grabbed guaranteed due to hit lag/etc. how dumb is this? Extremely dumb. This binary situation actually ends up being a 90:10 advantage in ICs favour in both scenarios, against every character in the cast.
This wouldn't be too bad except that Popo can solo chain grab most of the cast, which is how their binary game plan turns into more like 99:1 unless you're personally at a high enough percent which popo cannot solo chain grab you (for most characters, this was like 0-80% being in this danger zone, lol). Popo will be able to stall long enough for nana to come along to finish off the 0-death.
Everything in Brawl turned ICs into uncounterable monsters.
I know this doesn't have much to do with footsies, as much as them really denying every character in the cast a neutral game/ability to play footsies. But yeah, if it wasn't for ICs I'd still love playing Brawl and would've been keen for the game's future even post-new game. I don't see any reason to go back now though, no matter how much depth that game actually had even with most character's dumb free jank, none of it compared to ICs. They solely are responsible for killing Brawl for the rest of those who continued to enjoy the game during the entirety of it's rise/decline.