I'm kind of a 3.5 grognard who drifted away from WotC, but if D&D Next is something I like, I'll go back to it (assuming I get back into a group again). And I think that there's a large percentage of people who left 4E who can be won back if the game is really cool. How much, I'm not sure - although, I know that there will be some people who can't be switched from 3.5/Pathfinder just as there may not be people who'll switch from 4E to D&D Next.
It seems obvious that there is some proportion of PF and OSR players who are not going to be recruited by WotC to play 5e. I don't know what that proportion is, but would be surprised if it's more than 60% of PFers. I'd be equally surprised if it's less than 20%. I hope for their sake the WotC does
have some idea.
It seems also fairly obvious to me that WotC does not want to risk losing a significant proportion of its customer base for the second time in 5 (or so) years. That's why, from WotC's point of view, it is surely imperative that 5e be attractive to many, if not most, of those who are currently paying WotC for 4e material.
Which then raises the question - why is 4e attractive to those who play it? and does WotC know the answer to that question? In retropsect, it seems that they didn't know the answer to the same question in respect of 3E.
To date, I find that their characterisation of 4e does not capture what I like about it. 4e, for me, is not primarily about tactical combat. It is about dramatic
combat, with the tactical format (squares, tokens, action economy etc) as the medium through which the drama is resolved. It's about a system that encourages cutting straight to the action - be it combat or non-combat - without action resolution mechanics that lead to bogging down in (what is for me) mere exploration.
The fact that 4e is easy to GM is important, but for me secondary. I think that GMing Basic D&D is pretty straightforward - monster design is pretty simple, for example, as the vast lists of home-made monsters in Dragon, White Dwarf etc illustrate; and action resolution is fast and loose. But I'm not playing Basic, because it doesn't have the mechanics that 4e does which background exploration while foregrounding conflict and dramatically-paced action resolution.
WotC is full of knowledgable designers. They must all think about this sort of stuff, and have views on it. Some of Mearls' thoughts are available through stuff he said on RPGnet and the like back before he went to WotC. They must have had this sort of stuff in mind when designing 4e, given that it pervades the design. But nothing in Legends and Lore over the past year, or the more recent designer comments, has spoken about these matters in relation to D&Dnext. I'd like to hear about them.