I think Marth having his fair landing lag put to 9 frames would be kinda not good for the meta.
In general I think about me getting fairs to fsmash even easier than before and have a sense of dread/regret.
So I'll give you that.
Marth once had everything and now has little though. Stuff like Marth v Mario/Luigi won't be suddenly going opposite directions assuming humble buffs.
Marth used to have better mobility specs than those characters, better grab range than those characters, better grab rewards/followups than those characters, better out of shield options. And he had mostly better frame data, range and cool down than those characters.
Range is homogenized a bit better in this game. Marth could be given more grab game and likely not have game shattering impact. Marth could have his mobility buffed and that would make him funner to play for us and intrinsically better but would really only scale with high/top level play.
Everything else, the obvious stuff, I think in moderation won't **** him.