I'd still like the traditional 3 book model, but I would divide the books into tiers (heroic, paragon, epic), so in the end there are 9 books. I know I know - heresy to suggest 9 books, but by the time you get PHB 1-3, DMG 1-2, and MM 1-x, you are getting close anyway.
The reason I want them by tier, like the BECMI set or Dragon Age RPG, is that it allows more breadth of content directed to the tier you are actually playing in. In the first PHB for 4e, for example, I would gladly have traded the power descriptions for levels 10-30 and the paragon paths and epic destinies for levels 1 to 10 of the classes that came out in PHB 2 and 3. Having to wait two or three years to play some of the classes was...not good. I know the goal for 5e is to have all classes that where in a PHB 1 to be in the first release. I hope the goal and the reality of physical page limitations and layout don't collide.
For the Monster Manual, playing in a heroic campaign means never really needing all the high level monsters. I'd trade the Titan, demons and gods for more critters I'll actually be using.
It also gives WOTC the time to really polish higher level play for another year or so after the initial release. Time to make a coarse correction if needed, and a place to add those optional rules that naturally come up in paragon or epic play - castle building, mass combat, the planes, etc.
Downside, I understand, is those groups that level quickly and get through the first 10 levels before the Paragon material is released are going to hit a content wall that may interrupt their campaign a little. And some people are just going to balk at the idea of buying nine books for the complete game, especially people like the OP that want one book.
I'd be happy to have a streamlined subset of rules in one volume that goes 1-20/30 for the crowd that doesn't need any options beyond the most classic DnD races, classes and Monsters though. Sort of a DnD lite.
Verys