While the tiers do differentiate in terms of ergonomics and usability features, what you want to keep in mind is that often, camera bodies between the tiers of entry-level and prosumer will share
the same sensor and processor. For example, the Canon 7D, 60D, and 550D more or less all have the same sensor and processor. If you look, they all have the same Digic processor generation (Digic 4, although the 7D has two of them), the same resolution (18mp), and the same iso setting range (100-6400, with 12800 pushed). [On the Nikon side, the D300S, D90, and D5000 do a similar sharing of processor/sensor]. Image quality from all three cameras will be pretty much identical. Image quality is
not part of what makes a higher-end camera more desirable (full frames aside)--it's all about the usability features like burst rate, build materials, viewfinder size, etc. But when it comes to the final image, put the same lens on both, and optically, you're pretty much on a level playing field.
Secondly, paying more for a camera body doesn't necessarily translate into keeping it any longer than you would had you gone for an entry level version. It's like computers and cellphones. You just get better high-end features you can enjoy for the time you keep the device, but generally, between
Moore's Law and feature creep, most folks end up upgrading within five years. Which is why older, used cameras can be a great bargain: it's not like they stopped being great cameras, just because something newer and shinier came along. New features are great, but maybe you don't actually need them (e.g., video). Wanting them, though, is a different matter.