I think I'd be pretty okay with either model, but I have a slight preference for the current way. While you say that the problem with the current model is that people who want to play with frost giants have to buy another book, assuming that frost giants are level 11+, then that's still the case with your model. A D&D that only covered levels 1-10 wouldn't have many giants, beholders, mind flayers, or a lot of the other monsters that makes it D&D.
Plus, as Umbran says, to revise the system as heavily as 4th edition did, it would be wise to have the full level progression plotted out. And, to spend all that development time and then have to wait for 1-2 years to see a return might be a difficult decision for even a large company like WoTC.
Plus, as Umbran says, to revise the system as heavily as 4th edition did, it would be wise to have the full level progression plotted out. And, to spend all that development time and then have to wait for 1-2 years to see a return might be a difficult decision for even a large company like WoTC.