That said, the prototypes in this cycle have been around since 1999/2000
The shift was practically overnight (well, the year between races) but from the 1997 24H of Le Mans to the 1998 24H of Le Mans, the field went from largely GT/GT Prototypes to largely LM Prototypes. Both of the below photos are of the front runners from those respective races on the first lap.
1997 - McLaren F1 GTR, Lotus GT-1, Porsche 911-GT1, Panoz Esperante GTR-1, Lister Storm (if you look at the full version of this photo you'll see the Dodge Viper GTS-R's, Saleen S281s and Porsche 993s in the background too, but I cropped those out)
1998 - Toyota GTone, Mercedes-Benz CLK-GTR, BMW V12 LM, Porsche 911-GT1, Porsche LMP1 and the faint glimpse of a Nissan R390 GT1:
That said, at least 1998 had
some GT cars out in contention, even if the field was greatly diminished from the previous year. Fast forward to 1999 and:
You don't even so much as
see a GT car until that first Viper all the way back in the 20s. From there it's mostly ex-works 993s that just 2 - 3 years before would have had a shot at winning the whole thing.
Sure I'll be sad to see LMPs go, but let's be honest, after a few years things get stale in their presence. It happened with the 956, it happened with the R8 and most recently it happened to the 919 after less than a handful of races. Supercar manufacturers participating on comparatively shoestring budgets will never stand a chance against bespoke motorsport subsidiaries formed merely to dominate everything they could lay their hands on. They'll come back when the time is right.