How has Apple got away with screwing up our iPhone 3G?

A couple of months ago the world waited with baited breath for the release of the OS4 upgrade for Apple's iPhone operating system.

I didn't rush to upgrade the OS on my 3G, but a day or two later when I synched my phone with iTunes, it installed the upgrade.

Since then the phone has run slower, some of the apps just quit or stop working - even Apples own apps. The phone hangs and generally performs worse than it did when I first got it.

In short, my User Experience has downgraded, not upgraded.

Sure, OS4 works fine in 3GS phones and of course on the latest iPhone, but why the heck did Apple release it for 3G iPhones as well?
Did they test it on 3G iPhones at all?
If they did, didn't they notice how it screwed up performance?
If they did notice it screwed up performance, why did they release it for the 3G iPhone?
And given the widespread problems that 3G iPhone users have reported, why hasn't Apple worked out an OS4 fix to rectify it?

I now have a phone that was delivered to me less than 18 months ago, which works worse than when I first got it entirely because of software made and delivered, in a closed system, by the manufacturer. If that's not an Apple screw-up, what is?