-"Do they spend the money on the books when it comes out?"
That is perhaps the single most important question 5E and WotC faces, even more important that how the game mechanics work. Game mechanics will be solid, of that I am sure. They may not appeal to everyone, but they will work. But the question that keeps coming back is, why should a person buy it? Actually, that is not the true question WotC faces, instead it is...will they buy it?
Realistically no one should buy 5E. There are sooooo many good fantasy rpgs out there already, from various editions of D&D, retro-clones, generic systems that can do everything and non-D&D fantasy rpgs that break the D&D mold. But D&D is D&D. It garners attention simply because it is D&D. And WotC needs to play on that (and they are to an extent).
There will be a large number of potential customers out there who are willing to buy 5E, if only to read another game system. If only to understand the latest version of D&D. If only to try it without committing. I believe 5E has the potential to make WotC a phat wad of money. But only if they price it correctly. A lot of people will not be buying 5E to be used as their chosen game system for the next 5 years, at least not initially. A lot of people will not buy 5E if they are required to buy 3 high priced books. That requires a level of commitment to the system that a lot of people are not willing to make.
WotC needs to price 5E to the casual and curious gamer. They need to price 5E to the people who are not committed to the system.
They can do this with a bare bones book that allows a person to play, but without any frills. Perhaps a "basic" set that only covers the first 10 levels. Then they can release, at the same time, the traditional 3 book set for those people who have made a commitment to the system. They need a price-point for the curious and another for the committed.
This way they can sell a lot of 5E to those who won't be playing 5E.