As a matter of fact, the prospect of the character being "fundamentally busted" or a character being very easy to make overpowered, has not been proven yet. It is something that I have been thinking about the character more and more as time goes on. There has never been a time period where the character has been OP, or received buffs that has elevated the characters to the next dimension, so we don't really have any concrete proof that the character's ease-of-making-him-overpowered even exists.
I agree with the broad strokes of this. "It's a good thing Mac is underpowered because otherwise he'd be overpowered" is the eating glue of smash theory. I do think it would be a trivial exercise to break
any character, and am pleased with the conservative patch approach, but you're precisely right that there is no real evidence to this supposed unique degeneracy of Mac. It's part of a broader trend of people clutching their pearls at any character with extreme strengths or weaknesses.
The only "time" that we had an era of Little Mac being "OP" was the early 3DS era. That was an era with a hilariously underdeveloped meta, using a limited control scheme. Remember, that was the time era where some players thought Bowser was the best character in the game (I'm not kidding), as well as people thinking that Robin and Bowser Jr. was high tier. People nowadays have the impression that Mac's nerfs in the early SSB4 meta was due to the character being a problem character during the early meta, when that was not the case whatsoever.
I've actually never heard anyone expressing this view.
He rose from the undeniable worst character prior to the patch, to still one of the worst characters but much more disputable.
I'm just repeating opinions I have repeated a dozen times before, but I think Mac was never in contention for worst, is constantly underrated in a Twitter/Reddit circlejerk, and is perpectually judged according to strict solo-viability criteria based on a homebrew timeout rule on a homebrew stage list in a single format of the game. It's madness.
Even
Ganon has people admit "Yeah, but free for alls, right?"
I am curious though: aside from an armor buff to the move, what are the changes you are thinking of that could be applied to the move?
Well, for starters, let's talk about that 32 frame minimum charge before release/cancel. 36 frames is a colossal amount of commitment, which precludes a lot of use cases for a weak move with so little armor. Among other factors, it cuts into a lot of the move's hypothetical use in recovery.
Second, while it's great that Mac can shield cancel the move (or air dodge), he can't jump out of it directly. This would make the move a relevant recovery mixup comparable slip counter, even with the existing frame commentment and armor. It's not
great (frankly it'd be mediocre), but Mac will take all the mediocre recovery options he can get. There are a few characters Mac would like to mix this hypothetical recovery path in against (alongside the normal side-b vs slip counter), and as you lowered the armor startup and/or increased the armor value, the list would grow.
Third, the endlag is pretty extreme. Uncharged is 45 frames, including the active frames but still. That's -34 on shield (gets better as you gain charge/damage/shieldstun, but not much), which is horrible. Normally you'd expect a move with such massive endlag to have not just abnormally low startup, abundant kill power, character dislocation, or something like Bury status--but
multiple of those. Instead we get a move with double the disadvantage of a Mac f-smash for half the damage. (And overall worse armor!)
You could cut
20 frames of endlag off the move and it wouldn't break it. He actually still wouldn't use it much, as long as the commitment and reward are the same.
The main thing is increasing the armor, so he has an anti-speedster-juggle tool that works against say Mario nair, bair, and dair--not just (a single) uair.
Edit:
It might be helpful to think of Straight Lunge as two moves, similar to Ryu Focus Attack--a defensive option, and a
seperate and optional weird followup attack.
Straight Lunge, like Focus Attack, is not a "good" defensive option. Spot dodge and even air dodge offer far more freedom and
actual intangibility instead of mere armor, and not even good armor at that. But both are super long duration and can be used while landing. These are critical gaps normal defensive tools don't fill, which is enough to make "not "good" options" actually great. In Ryu's case, Focus Attack's limited armor is
enough and comes out frame 1, so he finds plenty of use for it even without taking into account the mobility aspect. It's great that Mac can now use the defensive half of Straight Lunge separate from the attack, but it's not quite enough to fill in those gaps and be generally useful at its job like Focus Attack is.
Meanwhile the attack half is just a terrible move with no value, especially on a character with impossibly high standards for what a good ground move looks like. It's nice than you can reverse it, but it's kinda putting lipstick on a pig. Focus Attack's attack is situationally handy and works well with the defensive half, but Straight Lunge's attack is just a mathematical dud in any situation. (And unlike fixing up the defensive half, I doubt improving it would address any of Mac's pain points.