I am thinking that VR saw this happen in skiing, and dropped Epic pass prices for this reason. Pandemic rebound was the trigger, not the cause. So, as an unproven notion, ~15-20 years.
EDIT: My post above with specific years was not accidental. They're pre-Lance-domination. My contention is that US viewership isn't formed by exposure, it's formed by hero-story media storms. If we proceed on that notion, US hero-story-driven viewership has been falling off since what, 2006 if we count Flandis? Taking all these suppositions in combination, we should be seeing a canyoning of road bike viewership, well, now through 5 years from now.
I know I am digressing and probably hijacking a cycling thread, but I see this a lot in Italy too, I mean this "hero-story media" thing. Don't know for other countries but here too, and not only concerning cycling. Skiing as well, take hte Toma years. When AT was racing, the national TV broadcasting was going to the lenght of putting on hold other programs to show him, if need be. After AT stopped racing, skiers pratictioners stayed stable for some years, but hten started to decline. And TV coverage dropped (and viewers went down to an absolute minimum of "core numbers").
Cycling went the same route, when I was a child, duringthe month of May, everyone from 5 yrs old to 99 was talking about the Giro, and following it. Via any available media, radio broadcasting, newspapers, TV.
What worries me most, is not the amount of viewers, but, rather, how much the diminishing viewers base will reflect, in the mid to long term, in the numbers of pratictioners. That is more worrying. Until a certain pratictioner base, in any sport, is not reached, said sport cannot "self-sustain", need TV coverage. After a certains base number, the sport will, self feed, for a time, even without TC coverage...but then