Considering the absolute hell Destielliers would rain on the show, this was probably the best thing the writers could do. I don't ship Destiel and I will never believe that Dean "loved" Cas in any but a brotherly way. However, I can buy that Cas loved Dean. It also makes the "Dean you are so, so wonderful, the best person who ever lived" speech understandable. People do feel that way about love interests, especially ones that they don't actually get/have to live with on a day to day basis. Cas idealized Dean, so he saw Dean that way as opposed to how Dean really is, a flawed person. I would rather that they hadn't done the scene but if they HAD to, making Cas's love unrequited was the best way out.

I will add that the the worst fallout of Sera Gamble losing control of the show in season 7 was that Bob Singer was determined to bring back Cas. I am absolutely certain that Gamble intended Cas's death to be permanent. I don't think the show ever found a way to use the character after season 6.