you are correct in your understanding that there is no comprehensive coverage testing of hardware for any release (be it of the kernel or of any distro)
but if you think about it for a little bit you will see why this is not the case.
how much would it cost to buy one of every model machine that is produced this year? and then think about having to try and go back to machines produced in the past. Then think about trying to house and power all these machines.
and you really would have to have one of every machine due to the various oddball bugs in BIOS/firmware/etc that show up.
Microsoft doesn't have this problem because it is the hardware manufacturer's responsibility to be windows compatible, not Microsoft's responsibility to work with random hardware.
Unfortunantly Linux is not in such a dominant position, so Linux has to work around the bugs, but bugs are only found when they are tested on specific hardware.
the people who released the Intel drivers that are in the kernel used in 10.04 thought they had everything working (and I'm sure they tested it on a bunch of real hardware), but when it got out into the wild, it was discovered that a lot of hardware didn't quite work the way that Intel expected it to once it was wired into motherboards and various customized BIOS' were running it.
As for the problem of 'fixing' 10.04 without upgrading, this gets to the age-old problem of trying to backport only the 'right' things. the number of changes in each kernel release are massive (approaching 10,000 changes per release), deciding which changes need to be backported is an inexact science, and frequently the changes involved for major work end up being so large that it can cause more stability problems than it solves.
you could try running a newer kernel on your 10.04 box to see if that solves your problem, but for something as fundamental as this, I expect that it will take a new kernel to fix it (not just an updated version of the old kernel with backported fixes). Ubuntu may release an optional kernel upgrade for 10.04, but one of the "advantages" of a LTS/Enterprise release is that people don't want the kernel to change.
you can't have it both ways, you need to decide between 'no changes' and 'bugs get fixed'