I'm sure when Tron came out lots people said "practical effects will never disappear". And yet with few exceptions they mostly have.
They get to do things that humans can't do. They can entertain in entirely novel ways. Using them as a substitute for human actors is kinda the faster-horses-not-cars of movies.
We use CGI characters for inhuman effects, but I think it's often not very effective. I don't care for most superhero movies because I don't really care about watching CGIs punch CGIs harder than human actors could do. There aren't any stakes; there's no dramatic risk. I'd rather watch a martial arts movie, which is limited by what humans are capable of.
That's still something we're learning. Part of what made the Lord of the Rings films more interesting than the Hobbit follow-ons was that the latter used more practical effects. New Zealand was a big star in LotR; Hobbit used more CGI backgrounds. Audiences felt the sense of place.
We'll improve the tech so that it seems even more realistic. The new technology used in The Mandalorian is really exciting (though I'd be curious to see how it played on a big screen). But it was at its best when it made up very real-world seeming worlds, while animation uses digital worlds in totally unrealistic and even more exciting ways.
So I'm expecting that we'll continue to have practical effects in the best movies starring humans, and continue to have human stars. But we're still waiting for the first mega-hit CGI star who can do whatever magic it is Robert Downey Jr. does. When they can catch that, RDJ may well be out of a job -- not replaced, but the entire enterprise gone.
Highly-paid actors are celebrities. They are engaging in a way that is incredibly hard to describe but nonetheless seems to be widely recognized. They aren't necessarily "good actors" (a term that is itself hard to define), but people enjoy looking at them. People go to a film just because some well-known actor is in it, and that's what makes them so valuable.
Every film set is chock full of talented people. Plenty of people could have done great work as, say, Tony Stark. Hollywood is full of beautiful and talented women who just never manage to get that "it factor". To displace highly-paid actors they'll have to replicate that, and that'll open up a whole new set of questions.
The ones who go first will be the low-paid actors who surround them. They are the ones doing sitcom pilots that get shelved, or commercials for dog food and allergy medicine, or being saved from having a bus dropped on them. Such actors are actually reasonably well paid on the days the work (over $1,000 a day for big-budget films or TV; $9,064 for a week as a guest star), though most of them work only a tiny fraction of the time. There are more people making a living at that than being a star, though the odds still suck.