I see it as this: everything they’ve done up to 2024 in this film is already done on this MCU timeline and according to the rules established earlier in the film are set in stone in our present (hence why the plot revolves around time travel to retrieve the stones and return to the future and not to just kill Thanos in the past instead). so when Steve goes to return the stones, he stays in 1945 to live the life he missed out on with peggy, and keeps his 1945 self from being frozen in the first place, allowing the past Steve Rogers to live a normal life, being older and deliver the message after everything enfolds. In this theory, it wouldn’t break the time travel rules set up earlier, and is sorta hinted at when Cap fights himself and the Bucky knowledge gets the attention of past cap.

That’s my theory anyways.

I see it as this: everything they’ve done up to 2024 in this film is already done on this MCU timeline and according to the rules established earlier in the film are set in stone in our present (hence why the plot revolves around time travel to retrieve the stones and return to the future and not to just kill Thanos in the past instead). So when Steve goes to return the stones, he stays in 1945 to live the life he missed out on with Peggy, and keeps his 1945 self from being frozen in the first place, allowing the past Steve Rogers to live a normal life, being older and deliver the message after everything enfolds. In this theory, it wouldn’t break the time travel rules set up earlier, and is sorta hinted at when Cap fights himself and the Bucky knowledge gets the attention of past Cap.

That’s my theory anyways.