Well, apart from not appearing in any of his post-1970 movies...
He would probably become the subject of some pretty severe canonization, in the manner of James Dean, since his two most memorable recent roles will have been youth counterculture classics The Graduate and Midnight Cowboy. His image would appear on a lot of posters in teenagers' bedrooms and school lockers(mostly as Ratso, I think, since that was the "edgier" role). He obviously never has the chance to develop into the more straitlaced, middle-aged dork persona that I think was visible in his roles as early as Marathon Man.
The Weather Underground gets a pretty bad rap among its target demographic, for having killed(even accidentally) such a celebrated symbol of youthful energy and rebellion. Conservatives, on the other hand, have a good chortle about "one goddamed hippie getting himself blown up by some others".