On the contrary I think that Fantastic 4 has been very good for quite some time, however like many titles 'good' does not necessarily equate to 'popular'.
I dont think Marvel will destroy the legacy of the Marvel Universe, although I do think it will be significantly altered.
Characters origins will be shifted in time - so Stark will have always been injured in Afghanistan, the Punisher will have always been a Gulf Veteran, Hulk and the F4 wont have received their powers as a result of the cold war arms/space race, Nick Fury will have always been black etc.
Alot of peripheral characters will probably cease to exist in the short term, perhaps to be introduced at a later date as and when needed.
I also think it is inevitable that the comics universe will be brought more in line with the cinematic universe (I actually think that is the main principle behind the whole change) in order to try and attract in some of those millions of movie fans who dont currently read comics. I dont think it will actually work, but thats a different debate.
Most of Marvels main characters will still be easily recognised as those we currently know and love, but with various differences - most subtle, but perhaps some a little more drastic.
Continuity will however essentially cease to exist. I have no doubt that everything that has happened in 75 years of Marvel comics so far will be essentially erased and consigned as having never happened in the first place as the publisher attempts to write a new, more streamline, more convenient, more profitable history for itself.
Personally i think its very sad, and i am already satisfied that I will not be maintaining my current very significant investment in Marvels comics. Certainly very few of the Secret Wars titles interest me.
I'll be picking up a lot of this Secret Wars stuff, mostly because it looks like it will be a good end cap to my collection. We have to wait until we read it, of course, but in my mind, this is how the Marvel Universe ends.
From here on out, I'll probably be putting lots of money into back issues.