Surely you jest. The back button has seen significant overhaul in the time since it was introduced; it changed from activity-to-activity navigation along the single "back stack" to a confusing, circuitous path walk between applications, their "up" stacks, their "back" stacks, and everything else under the sun. It's difficult to determine when it will take you to the previous activity or to a page you haven't seen in the days since you last launched the application you just ended up on.

If you're in a browser, the back hardware key takes you back -- until it determines that you've reached a navigation root (which might not be the earliest page in the chain), and it pops you back to the previous application or the home screen. Perhaps it closes the tab?

If the keyboard is up, it dismisses it -- unless you're in an autocorrect action (* on certain phones). Maybe it opens the keyboard, too, because it took you back to a previously focused field.

It's far from "not overloaded with other meaning based on context". It is, in fact, "entirely overloaded with other meaning; much of which is based solely on context."