Or, you know, we're tired of the super crunchy version of D&D that is Pathfinder. Don't underestimate the power of system fatigue. It is is hard game to run, especially the way the APs are written -- important info is buried in the text. This is on purpose (I know because I asked) because Paizo knows many of their customers enjoy reading the APs and don't necessarily play them.
People who don't run the game are still a valid market, of course, but a complex one to target. The real area of potential conflict is where people do run the game, and are, as you say, tired of PF's complexity. The thing is, are those people willing to give up Paizo's tightly-themed, largely well-designed, extremely numerous APs for the single WotC AP for 5E (which isn't even an actual AP yet), when WotC have a long history of crummy/questionable APs, just in order to get lighter rules?
My guess is no, for the most part.
Especially as there are those among the PF community who actually like the crunch.
So this leaves 5E in a tricky place. They can't guarantee to get PF people back, because the pull of APs is very strong. They can't guarantee to get 4E people back because, well, they're not supporting a lot of the stuff people liked about 4E. They can't guarantee to get 3.XE/PF people back, if those people like crunch, because 5E is, superficially at least, anti-crunch. They can't guarantee to get OSR people back, because 5E is much more complex than most OSR games.
So... definitely a tricky one. I suspect good marketing will mean a stronger initial release than 4E (and 4E wasn't terrible at release), but it will then be up to WotC to hold on to those customers, and I don't think that's going to happen unless they get certain things in place.
As noted though, maybe the actual game is only the small part of the D&D brand, so maybe they can fail to get back PF/OSR/4E customers, for the most part, but still do great through branded stuff.