At this point, I'm not even talking about them reforming in a new company to make oldschool adventure games again. They could have used their production know-how to make other sorts of games. (Obsidian didn't exactly go on to make isometric RPGs throughout the 2000s, after all.) There was a lot of institutional knowledge at Sierra that just got flushed down the toilet.
Who are you talking about? Many Sierra developers did continue making games. I assumed you were specifically talking about the famous project leads. Among them, I know that Mark Crowe went to Pipeworks, Jane Jensen took a five-year hiatus to write novels (and then returned to game work), Jim Walls (PQ) went to Westwood (after working at a couple Sierra splinters that never amounted to anything), the Coles went to Legend to make Shanarra, etc. So, I dunno, I think maybe the premise of your question is wrong? (BTW, I may be misunderstanding or missing something, not trying to be argumentative.)