How Loki Fixes Avengers Endgame’s Biggest Plot Hole
A "Negligent" Captain America
In conclusion, the problem many people had with "a Captain America just sitting around between the 1940s and now" isn't really the case, and honestly never was.
After Cap places the stones back in their right places and shares that kiss with Peggy, he's no longer Captain America - he's Steve Rogers, a soldier who finally managed to actually live a life, find love and do everything the whole rest of his existence was ignoring.
He hates bullies, it's why he signed up to take on the biggest bullies of the time in the 40s, and this Rogers knows that in the end, the most chaotic tyrant in the cosmos will always be taken care of, placing him back in the past and freeing up his responsibilities.
I mentioned how Agent Carter kept Peggy's partner a mystery, but writers Christopher Markus and Stephen McFeely added fuel to this notion of Steve gracefully growing old when discussing Civil War. Following fans pointing to an old man carrying the back of Peggy Carter's casket in the 2016 movie, Markus noted at Comic-Con that "Stephen and I are just so taken with the idea that Steve went back and got to live his life".
To bring Endgame's rules back in again, there's enough here to say that the movie allows for this, alongside what we've learned in Loki.
Bruce explains that "the past becomes your future", i.e. Cap's life always led him back in time as we perceive it, but this is the way things always had to go. After the ice, after everything that happened in the 2010s, one day Steve is in 2023, the next it's the 1940s - that is how his life always played out, we and he just hadn't seen it yet.
To bring more wider context and canon from Loki in, it's clear a Cap constantly on the run from the TVA, who accidentally stumbled onto the fact that all time as we know it is only part of an infinite number of realities... didn't happen. Chances are he'd tell someone in that 70 year gap, or he'd have something to say other than calmly passing on the shield to Falcon on that fateful day outside Avengers HQ.
Either way, Loki's new canon-expanding ruleset regarding multiversal anomalies, variants and timelines retroactively sews up Avengers: Endgame's biggest plot hole.
... or does it??