Interesting point someone made on Twitter:
Yamaha made several vocaloids with famous VPs, partnered with large companies, and brought old vocaloids back from the dead for V4.
Almost all of them tanked.
Is it any surprise they don't seem to care too much anymore? No matter what they do, people will just buy Miku again anyway.
Ah, yeah, well. As myself and a few others pointed out on that very same tweet, all of these instances were unfortunate, in some way. Unity-chan being used in Unity was Very Questionable Quality, and her design was. A choice. (Should've gone with Akaza as the main design, if you ask me). Arsloid was also strange, where his normal bank was okay, but his other banks were very clearly made just for XSY as they were much choppier, and he was pretty grating all around. Sachiko ended up being even more niche than Tonio is, somewhere in the land of Prima -- she sounds great when used to her strengths, but doesn't really fit the popular music styles that Vocaloid users tend to use. Fukase was the best of the bunch, and I'll admit I'm not really sure why he isn't more popular. His English bank is a little more weak/accented than I expected based off irl Fukase's singing, and the design is wacky + apparently his name/voice/image is very hard to get license to use? So maybe some combination of those.
I'm not saying these are all terrible banks or anything. As the original poster of that tweet said, Unity-chan can sound really nice if you put the work in, and I've seen a few Arsloid stans. Most people at least like Grandmaloid, and I know I'll be buying Fukase and joining his hype train someday soon. But looking at each individual case, I can see some reasons why each one may have broken down.
Also Yamaha just generally isn't great with advertising and continuing to promote things, and idk if that's a widely-agreed-upon statement or not, but I stand by it. Case in point, Cyman being dropped on Halloween morning randomly with no prior announcement and then essentially never advertised outside the "buy V4/Cyman/Cyva for $250!" bundle that happened, and now "oh yeah he and Cyva/VY1/VY2 come with V5 Premium!".