Also, we must take into calculation that today games take much longer to develop than 10-20 years ago (look at Elder Scrolls, GTA and other)
Those two games are poor examples, in my opinion, as they are both known for constant rereleases simply to milk money.
Having said that, there are multiple reasons why the break between Civ VI and Civ VII could be longer than between previous editions. As you mentioned, games
do indeed take longer to make than they once did and as OP mentioned covid likely slowed things down. In addition to that, Civ VI has had a longer support cycle (2 expansions, six DLCs, New Frontier Pass) than Civ IV (2 expansions) or Civ V (2 expansions and a few DLCs), which likely delayed Firaxis starting work on Civ VII, and if I'm not mistaken Firaxis also released another, unrelated game last year or something.
And that's ignoring that if Civ VII were to be announced tomorrow, which from our perspective is just as likely as it being announced any other day this year or the next or even the one after (assuming no promotional material beforehand), it would still be in time for a release that would create an equal gap as the one between Civ V and Civ VI.
If you want to know when I'd get worried about whether Civ VII will actually happen, which is the core consideration of this thread? If there's no announcement by the start of 2024, I'd say. But unless Firaxis as a company goes out of business, I do not see any reason why the Civilization series would be abandoned, as it is their flagship title and may indeed be the most popular turn-based strategy series in the world. They will continue to work either on new content for existing games, spin-off games with similar gameplay that can be marketed to the existing audience of the game, or new games in the main series.
Personally, my money is on Civ VII right now. If they were working on new content for CIv VI, we'd have heard something; I believe the latest content/patch was something like a year ago, and that's too long of a radio silence for a game in active development. And if they were working on a spin-off that isn't yet far enough along to be announced, that would imply they
aren't yet working on Civ VII, a game that'll likely be in development for at least two or three years before it's release, which would make the gaps between releases in the main series longer than I think Firaxis (or, for that matter, 2k) is willing to accept.
(god I'm so bad at writing brief posts...)