Vocacolle is usually twice a year!
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It's definitely interesting how some of the Great Eight have remained relevant, whereas others like Meiko have really plummeted (her Vocacolle originals were in the single digits). We often talk about Crypton's software stagnation in terms of Miku, but I think it's affected their other products more.
In my opinion, the reason why Luka, Kaito, Meiko and Gackpo are no longer as mainstream boils down to the same few factors: they're over a decade old without much of a price drop to match, and their voicetypes now have cheaper, more versatile, easier-to-use competitors on more modern engines. Those voicetypes themselves are both quite common and not super sort after by Japanese producers.
If I were to take a wild guess, I'd say people who would have once chosen those voices are now using:
Luka -> Split off between IA/Chifuyu/any number of mature soft letter voices
Kaito -> Lost his casual users to Ryo, retained hardcore Kaito fans
Meiko -> Chis-A for her rough but mature timbre. Mai for people who just wanted 'normal woman singing'.
Gackpo -> Similar situation to Kaito, compounded by the fact I think his audience was being threatened by VY2 before we even got here
Miku and the Kagamines are too iconic to the software to fall from grace, Rin is also a distinct variant of Japan's popular 'cute but strong' timbre. And if people already own Rin, then they might as well use Len when they need a male voice. Gumi subsists on technical merit as a highly versatile product updated for multiple platforms.