In-N-Out is a low-margin, high-volume business, kind of like how Walmart is.
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 was that and more first. They established the template that In-N-Out continues to follow, as anyone older than ~30 remembers.
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.
We agree mostly. I'm sure they would never "give back" the labor spend they've already 'learned to live without.' Though if you cut minimum wage, it'd suddenly become possible for a new entrant to open a restaurant that would embarrass them, by having twice the staff while holding other things similar. Today that's impossible to do without losing a ton of money, so the current businesses are protected from competition (other than from those "cult" type of place like In-N-N-Out. But how much more business can McDonald's really lose to In-N-Out, since they're basically running at capacity).