Do you really consider SC2 a screwup? Remember, SC1 was far from perfect as well when it came out. The only blizzard game that was perfect right from release IMO was warcraft 3. SC2 might not be their best, but it is still a really great game. So I can't really follow that SC2 bashing.
Can't talk about Diablo3 though, that series never interested me.
SC2 wasn't terrible. I think a lot of people were angry (and still are) about the state of custom games, which were a lot of fun for a lot of people. The implementation in pre-Arcade-patch SC2 was so bad that only ~15 maps were ever played. Post-Arcade is better but still misses the mark in several key areas.
I think it shows that (a) the staff is being manipulated by upper management, who doesn't understand what made Blizzard games good in the past or (b) the turnover has been so high in this industry that the designers and developers themselves don't know what made these games great.
Can't talk about Diablo3 though, that series never interested me.