If the C6 in '04 is just a body update with the same basic platform, then they will have produced close to 250,000 C5 units. The Corvette has sold better than expectations and the tooling will have been fully amortized.
The C3 was a new body on the C2 platform and it was done after only about 100,000 C2 units and was originally planned for '67, but was delayed due to technical problems with the T-tops.
GM presents the C5 as a world class sports car. The performance is certainly there, but it needs a "manuamatic" type gearbox - not for me, but that's the way most customers are ordering their Ferraris. It also needs a styling update, which should include high tech fixed headlamps.
In good economic times, sports cars sell well, but they sell terribly in a bad economy. The recession of the early ninties is what delayed the C5. I think it was originally planned for '93 at which time the C4 was nearly ten years old.
If the economy never went into recession we could expect a new C5 platform about every 10-12 years with a major update about half way through, but given the unpredictability of economic conditions more than a few months out, trying to predict the long run cycles of the Corvette is a crap shoot.
The $64,000 question now is will the C6 be delayed if the current economy sinks into a significant recession. All the auto companies are going to be seeing red and may be looking for ways to cut expenses. If the C6 is truly in the schedule for '04 then tooling and parts contacts will have to be let this year. We'll all just have to wait and see how the situation develops.