Is metal dead?

but I'm not sure about the next generation, if they see all sorts of rappers and so on on TV from childhood.

I myself listened to all sorts of nonsense until I was 11-12 years old, and then my cousins got me into metal:)
Although, at the same time with this "nonsense" I listened to Lordi, so I already had a love for metal back then :)

And in what year did they gain worldwide fame?

And what does that have to do with it? You yourself said "And how many of them appeared in the 70s-90s?". :) Well, for your information, Slipknot in 1999-2000 :)

And how much time did it take them for that?

One studio album :smile04:
By the way, Bullet for My Valentine is a good example of a "very popular band of the 21st century" :)

P.S. Popularity doesn't necessarily mean quality.
Don't judge a band by its cover :D