I agree that the capability will be there. but it will still cost time and money to model the characters. You can't just press K for Kirk or S for Spock. Somebody has to model and animate these things.
Pulling some old archive footage would still be cheaper.
You severely underestimate the progress that will be made in 20-40 years, while severely overestimating the expense at which it will come. I urge you to look up Moore's Law, and contemplate its implications for the topic.
Bookmark this post and check back in 20-40 years: movies will look like "live action", but with CG (non-existent, or long dead) actors. Forget about reboots of old movies. We'll see new movies from the "original" actors. A whole new "Road To" series with Crosby and Hope. The long-awaited sequel to "Gone with the Wind" with the original cast. The 4th and 5th season of TOS. etc...
Look only to the popularity of resurgence in musical genres. Everything old is eventually new again. The technology dictates how intense (and "real") that return is.
I guarantee this will be the future fad.