Sure you may believe Messi is the GOAT, but Ronaldo's name also comes up in the discussion.
Cristiano is not going to disappear just like Pele/Messi/Maradona will not disappear. Even players one level below like Di Stefano, Beckanbauer and Cruyff are not going to 'disappear'.
You are overstating United's permanence in the tier that contains Barca/Bayern/Madrid, You had a chance to get in there in 2010 but you squandered it with poor signings. The new generation of young fans (especially non-English) are idolizing City/Chelsea/Madrid/Barca a lot more than United.
Even your most reputed players - Lukaku (grew up following Chelsea), Pogba (literally United youth) don't consider it unthinkable to transfer away.