...why shoudn't one look at different editions of D&D as an upgrade?
Well, as others have already mentioned - an "upgrade" is an improvement. If you are talking about computer hardware, you have some point of performance or capability that you're talking about when you upgrade.
With an RPG, there is no universally accepted, or ever generally understood, performance points that everyone cares about, such that a change in edition is necessarily an upgrade.
I find that 4e combat, even for a neophyte, runs more smoothly than 3e combat. However, 4e combats seem to take longer than 4e. And 4e is a bit lacking in support for non-combat related powers.
End result - is it an upgade? That depends - if smooth and simple combat were my basic desire, then yet. Otherwise, it may not be.