Correct, they will never stop doing it. But they will eventually start seperating it more, letting the user chose to use the standard home screen and dialer for example. Not sure of the Sense UI but it shouldn't be no problem to make it just another (preinstalled) alternate home screen where the user can chose to switch to the standard home.

Also work on the UI side hopefully also means faster portation of those UIs. Not sure why it takes Motorola/HTC/Sony so long to port their versions to the latest Android code, but i guess this can be improved on the Android side of things, too.

Agreed: why would they drop them now, unless they were just startlingly worse? They aren't interested in Android's success outside of their devices, they want to differentiate themselves.

Even if they are startlingly worse (motoblur seems pretty universally hated, and Sense seems to strongly polarize, some people find it very nice, others find it awful and full of useless gradients and badly polished looks), I doubt they're going to drop them.

And this is terrible for Android as a platform, in my opinion worse than the issue of hardware fragmentation and maybe even worse than the handsets left in the dust re. OS updates, because it means UI knowledge becomes non-portable from one Android handset to the next, which is horrible.