I don't know if you can say it's been dead for decades, but surely, as stated before, the general motorsport fan may have grown into liking other kinds of motorsports.
Same goes for motocross racing. Actually watched a travis pastrana video yesterday where I saw him race, and remembered that was a thing. Probably still is a thing, but I have surely not been keeping up. I think the same goes for F1 for alot of people.
Why would MotoGP be the only sport worth watching though? I'd much rather watch something like nascar or D1GP. But hey, that's me.