...I never thought about this possibility of it being all just a fad. I think it could explain a thing or two I've been wondering about. I don't think it's just a fad (at least in Japan, I have no fucking clue about the West) but I honestly think when the boost is all said and done, Kimetsu will have reached its absolute peak. It will be an extremely impressive one considering where it started, but still a peak. I do think Kimetsu will retain some new hardcore fans, though, so even when it drops in sales, it'll still have made an astounding run nonetheless.
That's just my take, at least. Who the heck knows at this point?
You say that, but the reason that The Seven Deadly Sins Season 3 is what it is because the anime movie, Prisoners of the Sky, underperformed (compare box office numbers of that to MHA: Two Heroes which came out around the same time) and so Aniplex themselves just dumped the IP. Now, Kimetsu is a huge success, so that makes this far less likely to happen, but never say never.
I do hope Aniplex allows Ufotable to adapt the series in its entirety though. They should see that it's been working out out remarkably well for them.