This is why time travel stuff is fun!
I agree that Cap travelling to the past to have the chance to "live his life" ought to create a divergent timeline, since there's no other way to account for there always being 2 Steves somewhere from when he arrives until he leaves to put the Stones back where they came from.
...But...
It is possible the MCU we have seen play out throughout all the movies
is a "divergent timeline" already. That is, there was a prior timeline we didn't see where Peggy didn't get married, and Cap just didn't show up after taking the Stones away. When post-Endgame Steve showed up in the 1940's, it created the MCU timeline that we saw play out. He was always there in the past, he was just never called out to the viewer. In that sense, he
has to go back and live his life to make sure the timeline remains the way we saw it originally.
The other interesting time artifact out there is that when 2014 Thanos jumps into the Endgame timeline and is killed, it creates another "divergent timeline" where he disappeared one day and thus never gathered the Stones or did the Snap (that universe will also never have a Guardians of the Galaxy team, since Gamora disappeared too.) But that timeline
will have a Black Widow that survives, because she will never need to go back and sacrifice herself to get the Soul Stone (If we keep looking at it, we even see that
our Black Widow died in that Thanos-less timeline, presumably minutes before Cap showed up to return the Soul Stone to Red Skull...that would be an interesting scene, huh?)
I haven't followed any of the comics since the 90's, but it seemed at the time, especially in books like "Quasar," the Multiverse was becoming a little too well-traveled and mapped out, so I'm not surprised they tried to find a way to simplify things.