This is what will happem...
Actually, now is a great time to still get a good C4, especailly the perferred 94-96 variety.
Just like the C3, you will have different versions that are worth more. I.e., the L-88 vs. a regular 76' 350, etc.
It will become more and more of a sellers market (or go up in value) as the cars become older and more rare.
Those that choose to keep their cars in stock original condition will benefit in the future by doing so. Just like previous models.
special versions like the 96 grand sport, or a 94-95 ZR-1 will command higher prices because of their respective rarity.
Also, one thing that I think will make the 94-96 a much better value compared to others, is that GM did a better job of getting the bugs out of these models in their stock form, plus the engine is more advanced and allows easier mods to get more horsepower. These models also have airbags where as earlier C4's don't.
But I don't think the C5 or the coming C6 will affect the prices at all. One buys a C4 for typically the following reasons. It currently is the best value for a Vette and the post 93 models are fairly clean from an engineering standpoint. Or one just likes the C4 because of it's ability to make a smoker out of a Vette without giving up much reliability. About the only thing negative regarding C4's is at this time it's getting harder to find 1-owner cars to buy. Also, with the relativly cheaper price curretnly for the C4, you find the C4 being owned by younger groups (i.e. teenagers) who unfortunatly destroy them. Thus, you don't want to buy one of these cars (unless you get it really cheap). But the kids will actually help increase the value for those of us who are more on the collector side, because they will diminish the supply. This is similar to what happened to the SS396 Chevelle, 68-69 Roadrunners etc. They used to be all over the place and everybody had one. But they were wasted ( I even remember seeing a 69 RR used in a car bash to raise money), and now they are rare, making them worth way more.
In time, the C4 will increase dramatically and isn't influenedced by previous or successive cars.