While I agree that some things are fine as is as changing them would be a little extreme, most of the changes people want wouldn't make Samus top 5 or easy to play.
During development Sakurai and his team probably thought that her tools were too effective for her playstyle and nerfed her, but it feels like they went a little too far. Some of her moves extend her hurtbox way too much for no reason. That's a design flaw which could easily be fixed by reducing them, not removing them entirely.
Down tilt no longer kills but is unreliable in setting up combos. Her CQC is meant to be bad, but the added Ftilt sourspot hurts us even more. Zair now has 8 frames of landing lag but is the second weakest Zair after Lucas. Her grab game is weak, none of her throws come close to killing, down throw can easily be DI'ed, forward and back throw launches them too high for gimping to be very effective and up throw is mediocre. Her missiles get beat out by anything, not just clankable projectiles. Moves as weak as Sheik's Fair and Mario's Nair should trade, not outright beat them.
Making Dash attack hit earlier would just end up making hitting with it easier. If it hit earlier they would be able to get out hitstun just as Samus ends the move, as opposed to her current Dash attack which sets up combos because it hits while the move is ending. Having down air's hitbox match its animation is just making it visually consistent. Up air is fine as is, but her other multi hit moves like Fair, Up B and Up Smash have no reason to miss and whiff as often as they do, and it's never a pleasant experience getting punished for a move failing to work. If she could get the Falco treatment and have those moves autolink it would fix her.
If her moves were more consistent and some of her weaknesses were addressed she still wouldn't be top 5. She would still have hard matchups and because of her playstyle she would have problems with extreme rushdown. Samus just needs fixing. Samus is in no way the worst character in the game, but she does need help to get her out of the slump she's in.