This is not the case for us. We have two, older established sites that we have not made any changes to, that are suffering this dramatic loss of pages. We have lost around 10,000 pages on one and around 5,000 pages on the other. The pages lost are primarily from discussion boards. Our positions for our main key words are holding well, but we used to get a lot of traffic from more obscure searches that came through indexed discussion board pages.
Strangely we have another site, in a different sector, with a discussion board that seems to show an inflated page count now, but perhaps the pages were there but never properly indexed previously.