Well he was a fantastic actor in those 3 movies, pop icon to this day, widely considered "the best actor that died too young"  yada yada and all that. 
Im not defending this btw, its shit and insulting to every actor currently working. Somehow I feel if you were to do this with John Cazale people in the indurstry would have a more public outrage. 

This is kinda shitty tbh. The fact that raw, authentic talent can be replaced with CGI and stand-ins makes movies feel so much less personal to me. When an actor really tries, they can bring a whole new perspective to a film that makes it feel so much more intimate and unique. Shit like this actively discourages experimentation with source material and is basically the equivalent of typecasting an actor for a specific type of character. Not to mention that this is insulting to actors who are genuinely trying to succeed in the industry and James Dean himself who is forced to be associated with a project he did not consent to.