Well, I've not heard that. I did go to the IMSA race in May; to one of the world's top historic races, the Motorsports Reunion (where Zak Brown races in early McLaren F1 cars) a couple of weeks ago; and will be at the Indy care season finale race there next weekend. Porsche brings their Rennsport Reunion there every few years which draws about 90,000 as does the Motorsport Reunion. The IMSA series is very similar to WEC and includes the 24 hours of Daytona and the 12 hours of Sebring. Those seem pretty major and I've heard nothing about them being cancelled in the future.
But I don't follow motor cycle racing but I guess they did lose MotoGP and now have something called MotoAmerica Superbike.
But what the 3 tracks really lack for F1 races (some of which could be addressed with $$) are on-property facilities like adequate garages for F1, on-site medical facilities including a safe place to operate a medevac helicopter, adequate hospitality spaces and grand stands and parking for 200,000. I think the tracks themselves would be fine in terms of width, run-off and safety, compared to many other on F1 calendar. The track was resurfaced a couple years ago and improved to further meet FIA standards.