I'll jump in on this one - because RDJ knocked it out of the park in his first appearance - he MADE the MCU. (And I say that as someone who didn't fall in love with the MCU until Thor and Cap debuted.)
The problem with Riri (and America Chavez for that matter) is neither of them were anything other than generic (IMO) in their initial appearances. There was no "wow - I want to know who that is!" factor although I'd hoped there would be.
I'm very fond of Sam and quite liked Monica but neither made me say I NEED to see more of them (though I'm looking forward to both of their upcoming adventures).
Which is different than say, Shuri or (again IMO) Ms. Marvel - both of whom instantly clicked for me as characters I adored from the moment they were introduced.
You're always gonna have resistance to any replacement characters (politics/trolling aside). So they've got to WOW right out of the gate. And a lot of these new kids... aren't. Not totally the fault of the actors, I'll add, so much as scripts not servicing them well either.
So they can pay a discount for Riri and likely not fill a lot of seats. Or they can pay a premium for RDJ and likely fill them. It's up to Disney/Marvel what the cost/benefit analysis ultimately shows.
But RDJ, Hemsworth and Evans had star-making turns in those roles right out of the gate (along with Pratt, Boseman and others). Thus far, the next generation of MCU hasn't. And to me, that's the biggest difference between Phase 1 and Phase 4. Not fatigue - fandom. I'm looking for characters I instantly love and few of the newbies do that for me thus far.