Actually i can also follow your logic too. The slayer and knight could also heavily step on the PHB 1 fighter builds' toes, a one handed defender and a two-handed defender/striker hybrid which are more distinct frome each other than the two PHB 1 fighters (+1 bonus on their choice of weapons and maybe some powers)
Maybe they won´t invalidate the PHB 1 stuff, because of the lack of dailies and because they won´t have a different defender ability than combat challenge/combat superiority. And because this is widely seen as the strongest defender mechanism out there, there may be a lot going for the old fighter.
Right now, D&D 4e is very modular in its nature, MP2 builds are exchanging class abilities quite heavily... i don´t see older material fading out completely (if PHB gets a nice reprint)
Maybe some of the builds in MP2 were better packed in an essential product. I could have seen the archer warlord as an essential martial leader.