For comparison, EVE Online has 300,000 monthly active players. Call of Duty has 30 million. That implies that about 1% of gamers are interested in the sort of games Sid Meier is talking about. The demand for "complex video games" absolutely has diminished dramatically as a proportion of the gaming industry.
Even otherwise, the crux is that the entire gaming pie has grown substantially. There are lots of new genes and platforms which have become popular, but demand for games like Civ has grown as well.
There were about 50 people in my year at school playing pirated copies taken from one original. The kid with an external disk drive and a copy of Cyclone was very popular.
Today people expect quality, and something that caters to them. And you have plenty to choose from. That's why you cannot make game that everyone will play and enjoy (except of Portal, everyone loves Portal).
Games are widespread form of entertainment now. a 1% now is worth beyond imagination what 100% in 90s was worth.
In a way its easier for a niche game to be financially successful.
Of my friends, I think only 2 of us had actually purchased Starcraft licenses, lol. Everyone played it, and everyone loved it but ere'one didn't pay for it...
That is a big factor, especially with younger audiences, but ofc it doesn't explain 300k vs 30m.
Also the golden age of classic MMOs is kind of past, look at World of Warcraft numbers 8-13 years ago and compare them today, even when lumping all "fantasy" MMOs together.
But in the market for complex strategy games, Civ was absolutely king. But that's because the market was far less saturated at the time; nobody else had made a game like that before. Now there's thousands of games trying to do things like that, so it's much harder to stand out.