I really doubt it will kill Disco, for the same reason Guild Wars 2 barely put a dent in the juggernaut that is WoW - people get invested in things; all the time they spent grinding levels, raiding and capturing flags in Warsong Gulch keeps them attached to the game. All the lore, areas, etc. are familiar, and they probably have friends playing it too.
I think the same'll apply for disco. All the time spent hammering out RP on forums and in-game, all the trade routes ran to fund X project or Y ship, all that will probably keep people bouncing back for a long time to come, regardless of just how "next-gen" Star Citizen is.
That said, I'd be happy to be proven wrong and have Star Citizen be absolutely everything I've ever wanted in a spaceship game ever, but I think it's being overhyped in a lot of ways.