Actually, back in the days when no one cared about Arashi, I always thought they were meant to replace SMAP! Too bad all the old forums/posts are gone because I know I said multiple times that JE wanted them to be the next SMAP. I agree that SMAP was legendary for reasons other than just being crazy popular and Arashi won't have that but I think Arashi have become a legend in their own way. Arashi will be remembered for having a period of crazy popularity (like Morning Musume 10+ years ago).
Interesting you mention the replacement group thing. I agree TOKIO was meant to replace Otokogumi. I've always thought of KAT-TUN as Hikaru Genji, if not just for the clash of personalities (and yes, I predicted KAT-TUN wouldn't last and I was half right!). If anything, HSJ strikes me as a replacement for NewS (too many members thrown together like a tossed salad) and given they've already lost a member, they're on the same path! Kisumai is really just KAT-TUN ver.2 as well. I'm not sure about SZ (don't really know anything about them yet). It's nice to get a 5 member group (haha, I'll actually learn to tell them apart, I still only know like one member of Kisumai and that's because I can't stand him, lol).