I don't think that the everything has been done/nothing new concept has much to do with it. Honda-Tech is in the same state, still around but significantly decreased participation.
Honda-Tech was the largest Honda forum on the internet and even though just about everything had been done dozens of times over more than a decade (H-swaps, J-swaps, Honda engines in other vehicles, twin engines, mid engine RWD conversion, true mechanical AWD, front clip swaps, re-sleeved engines, frankenstein engine builds, stitch-welded unibodies, 600hp daily drivers) and half the vehicles were actually older than the owners were, it was still lively with an abundance of new content in every sub forum every day. The slow decline of internet forums, all internet forums not just CR-Zforum, has to do with the changing of the internet and increase and takeover of 'social'-media.
Even if nothing new was done modification wise (and we've only barely begun to see what can be done with the CR-Z as the most extensive vehicle modding is always done with less expensive used models and not on brand new purchased vehicles like the CR-Z) the place is still useful for the continued sharing of new information regarding longevity, problems, increasing maintenance, repairs and so on. But the CR-Z modding has only just begun.
Look at one of the most modified vehicles ever, 1992-2001 Honda Civic. They weren't extensively modded when new and only really hit their peak after discontinuation (like 10 years after discontinuation). Its the case with many vehicles. It only makes sense to go wild after the warranties are up and the vehicle isn't worth as much. Smart people aren't going to extensively or irreversibly modify leased or financed vehicles either.
A significant number of former CR-Z owners also purchased this car right before making a family so we will see new younger owners as the prices drop after selling and older prospective owners post sending kids to college and looking for a smaller more fun automobile.
Another worth mentioning is we could see a significant increase in CR-Z desirability with an increase in oil and gasoline prices, which on a long enough timeline is inevitable.