I'm 32, been listening to metal for over 20 years and know virtually nothing from Rammstein or anyone who likes them. I remember the song 'Du Hast' being popular whenever that album came out but nothing else after that. To say that they're at a popularity level that's close to Metallica the funniest thing i've ever read on here.
The reason they sold out MSG is quite simple. It has nothing to do with them being insanely popular at all. The fact is they haven't toured here in forever, they played MSG on a saturday night and it was their only US date announced. So you got fans from all over North America to make a weekend trip to NYC and see them. That's the reason they sold the place out.
They may have a good following but if this wasn't a special one time only performance they wouldn't have had a chance in hell of selling out MSG.