For the record, Enterprise didn't fail because it clung to 90's (really 60's since this is Star Trek we're talking about) era optimism in a pessimistic world. It failed because the characters were bland, the show spent 90% of the time being boring and sticking to the tired single episode stories that Deep Space 9 had the good sense to move away from, and because the producers and writing staff that were left with the series at that point weren't very good. Most, if not all, of the writers who had written some of the seminal TNG and DS9 episodes were pretty much gone after that show ended. Sure, it might have helped Enterprise a bit to address the sort of events happening in the real world at the time since good science fiction often is allegory for what's happening in the real world, but it was still, overall, just a bad show much like Voyager was.
And during all of that time we supposedly had nothing but pessimistic sci-fi, we had Stargate SG-1 and Atlantis being successful just being fairly fun and light hearted shows. And I wouldn't say that the balance of optimistic to pessimistic sci-fi even changed in the last decade or more, I think people just stood up and took notice because BSG was really damn good, and was something completely different from any sci-fi show in the decade that preceded it. That does not mean that Doctor Who came along and saved sci-fi from pessimism. It's just a fun show that's had a great run and balances the two pretty well. Nothing more to it than that really.