To me its as easy as using the infinity gauntlet to alter reality in a way that doesnt need details and explaining to explain the introduction and appearances of mutants and characters like Reed Richards.
Also opens the door for dudes like Idris Elba to get new and better roles in the MCU.
No time travel soft reboot or events occurring in a different part of the multiverse needed.
Basically what Marvel just did at the end of Secret Wars where the multiverse died and came back. Everything in the main Marvel universe's past still happened (Infinity Gauntlet, Civil War, Onslaught, etc.) but everything from the present onward is different (Miles Morales and co. in the main universe, the Maker, New Universe characters, etc.)