See, I think you're confusing one argument (that FR is the most well-known and most used setting) with a completely different point (that most people want another FR setting book).
Point 1 is indisputable; FR benefits from the video games as you say, but it benefits most from being the setting in nearly every hardcover adventure book for 5E. That alone will make it easily the most used and well-known setting used by 5E players, no question. Even Exandria can't match that.
But just because Point 1 is true, it does not mean that all of those users want another FR setting book. In fact, it could mean the exact opposite, and that those users want something other than what they have; they may ask for more material for gothic horror campaigns, or epic fantasy campaigns, or steampunk, whatever.
I suspect that WotC internal market research points to that same conclusion, that most 5E players want more setting books, but for material that is outside of FR's purview; that's why we have gotten settings for Ravnica, Eberron, and Theros, which are decidedly not FR. And if you look at the recent UA, they seem to be pointing to either Dark Sun or Planescape material.
I have no problem with this, as I personally prefer new settings to rehashing the same old settings again and again -- if it is an either/or choice. Couple that with the fact that I'm not faithful or dedicated to any single setting, plus limited exposure to Magic the Gathering, and I'm quite happy with the idea that the future of WotC setting books is probably going to be focused on new/old Magic worlds.
But I don't think the two are mutually exclusive. Unless WotC really believes that a new and updated FRCS wouldn't be financially lucrative, or that it would stall the backlog of planned projects, given all of the factors, it just makes more sense to publish than not to publish. IMO.
In other words, if I have to choose either new settings (including Magic) or old settings (FR, Greyhawk), I'll choose the former. But I'm not sure that it has to be one or the other. If they're going to publish three setting books every two years, my preference would be two new and one old -- a pattern we've seen over the last couple years that may continue in the near future. Of the old settings, personal preferences aside, I think FR, Planescape, and Dark Sun make the most sense, folllowed by a group that includes Greyhawk, Dragonlance, and Ravenloft, then a third group of Mystara, Spelljammer, and others.
And then it comes down to the fact that you can't do everything in limited amounts of time. Given that Planescape is more of a cap-setting and can be tied to a Manual of the Planes, I wouldn't include that in my hypothetical one old setting every two years, so maybe we see something like so:
2021: Forgotten Realms (or Dark Sun)
2022: Planescape/Manual of the Planes
2023: Dark Sun (or FR)
2025: One of Greyhawk/Ravenloft/Dragonlance
Etc. I don't think we'll ever see any of the "third tier" settings, but who knows.