The accusation isn't that Phoebe Waller-Bridge will put on the hat and go off for more adventures after Indy retires (or even dies) at the end of the film Ford is 80, he's done here, and that would be just fine (I'd prefer Ravenwood-centered prequels myself). The insane accusation, being made by arch-culture-warriors, is that the film will retroactively erase everything Indy ever did and replace him with Waller-Bridge in Raiders, Temple, etc. Which is a mad accusation.
Yes, with the Avengers actors aging or simply not interested in continuing to play their characters, Marvel is turning to the non-X-Men, Non-Fantastic-Four characters (the plans were made before the Fox acquisition). Several are female. Several, like Shang Chi, aren't. Old Hawkeye is still around, as is Thor and Ant-Man and Vision and Strange and Spidey and etc. Original Hulk is still very active - they just introduced his son and the rumor is that the rights situation is about to change and he'll get a solo picture in the coming years. What should matter is the quality of the character and performance, and at least new Hawkeye and new Black Widow are vast improvements on their predecessors. Anyway, these are all Marvel comic characters, some with very long histories (She-Hulk was Lee's creation from back in 1980) so take the issue up with Marvel, not the MCU.
And as much as fans hate to hear it, Ford utterly loathed playing Han.