In-N-Out allows us to discard this notion. They've maintained full staffing while simultaneously paying staff above-market rates (even in California). They only charge 30 cents more for a basic cheeseburger, and the burger is much higher quality, so even that is hard to attribute to wages.
McDonalds, of course, is 5% corp-owned flagships and 95% franchise "opportunities". They don't have customers, they have tenant farmers who work the land leased to them.
So many parties either directly exploiting others or being able to do it by taking significant cuts of revenue for questionable value...
They literally can't cut staff because they only make money due to the incredibly high number of meals they serve per hour. So they have to serve as efficiently as possible, which requires that fully-staffed kitchen and counter, or there wouldn't be enough time in the mealtime hours to do that volume otherwise. It's just as impossible for them to cut any staff as an airline could remove pilots from the cockpit, in that the whole operation would cease to function.
McDonald's and the rest are never going to be In-N-Out though because it would take years of operating at steep losses in the hopes people would notice how fast and tasty they'd become. I can't think of a mature company that's ever pulled off going from "boring mainstream place" to "cult favorite" like that.
McDonalds chose to aggressively enshittify, In-N-Out did not. You're probably right that McDonalds will never be that again, I don't think they even want to. If you cut minimum wage in half today, they'd keep the same staffing levels and the franchise owners would pocket the difference.
I sometimes have the urge to go to an In-N-Out, but the drive through line loops through their own parking lot and into the one next door. There are people standing outside the restaurant because the indoor dining line extends past the doors. I don't really want to sit in my car or stand in line for 30 minutes.
I used to go to In-N-Out weekly when I worked a late shift; the line after midnight is much shorter.
Don't underestimate the equally intense downward pressure on staffing that minimum-wage hikes have provided (for instance, in California). I won't speculate if it's been "worth it" since it'll devolve into an off-topic political debate, but basic economics does predict that since labor costs twice what it did 10 years ago, they'll do everything to minimize how much labor they buy. Couple that with what you said, basically, using "Moneyball" techniques to finesse everything with all these optimizations, we get the operations we see now in all retail establishments.
And even if there are states that haven't done the minimum wage changes, the markets that have are so big. The national businesses that have adapted to it have proven out their barebones-labor model there - why wouldn't they roll out most of the changes nationwide?
I think if someone launched a fast-food place or a drugstore in 1995 that operates with the type of skeleton crew they use today, it would fail overnight due to people having a choice. But today, since essentially 100% of retail operates this same way, they don't experience any pressure to do better.