Since Dustin was able to be such a HERO as Babe in Marathon man, he eventually had everything to be a convincent Rick Deckard in Blade Runner. Probably the chemistry with Rachel/Sean Young would be better, Dustin is a sooo, unexpected, seductive man... Greets from Mendoza, Gustavo.
I could see this as having worked out, but Harrison definitely has more of a rough edge to his appearance that made him fit the part better. Just my opinion. Also...there's no way they could have ever gotten anybody to do a better job at playing Roy Batty than Rutger Hauer. He makes the best crazy person.