I don't think having a year without material will make them fade into obscurity; they are incredibly popular already, they have talent and will no doubt fill their 12 months with tumultuous PR, interviews and the tour.
One Direction (albeit not winners and with arguably less talent) didn't release any material for around the same amount of time and yet have made an absolute fortune due to their appeal with a hugely profitable target market: the teeny girls!
Little Mix will have the same market and will slot into an industry currently lacking in a big female pop group aka The Girls Aloud market.
I'm not a fan of pop, r&b, urban or any crossovers but these girls are quite amiable and I can forsee them being around for a lot longer than any of the other winners.
Marcus - I like the retro/50s route he seemed to be taking with his music; if he pursues this in his own work I can see him standing out a little bit and having relative success (a la Olly Murs) but otherwise I think he'll join the likes of Rhydian, Steve Brookstein, Shane Ward, Leon Jackson, Andy Abrahams etc.