If the PHB stopped at level 10 the spell section would only include levels 1-5, as in b/x. Combined with the shorter class descriptions, let's say that frees up about 50-60 pages. With those pages you could include more races/classes/backgrounds, more lower level spells, or, my preferred option, a mini-dm primer and some more monster stat blocks, so that the PHB would include all you need to at least get started.
Then you could have an advanced book where the classes could go levels 11-20, feats, prestige classes, class variants. This would be more extensive and robust than anything in the current players handbook, and so worth the money for players wanting a more complex, higher level experience. More importantly, as many of you have been saying high level play involves a fundamentally different kind of gameplay than lower levels, and this book could give player options and dm advice and tools to make that happen (strongholds, high level sandboxes, planar adventures, guide to high level villains and lairs, etc).
I've never gotten to high levels, but for me levels where an existing ability simply scales up (for PCs or for monsters) is not as exciting (improved divine smite, higher cr destroy undead, multiple attacks, etc). In the fiction, you could be 10th level fighting frost giants, but if you abstract the math its very similar to being 3rd level fighting ogres.