The reason slots were used in the first place was to add extra cache. This meant the chip itself could have a smaller die size, although I guess they had problems getting cache chips... and besides, on-die cache is faster and therefore preferable. As for the assertion that slot will die out completely I am doubtful. There is still a demand for large cache proc's, and I seriously doubt either Intel or AMD are going to integrate 4MB of cache unto a chip (well... they might, then they can charge even more outrageous prices and of course with the chips where the full 4MB cache dosn't work they can sell it as 2MB or something...).
<< Neither AMD nor Intel is producing any more Slot versions of their latest chips >>
Intel still produces PIII Xeons in a slot package, as per my argument above some versions contain extra cache.