I know she still has plenty use and there will always be fans or producers using any Vocaloid, regardless of popularity.
I just used her as an example, because she used to be the 2nd most popular Vocaloid after Miku. Her usage has declined steadily since her heydays between 2009-2012. Vocaloid like Gumi, IA, Rin, Yukari, Flower and Una have already overtaken her. Some in usage or in popularity. Some in both.
Popular songs are important to maintain a Vocaloid's relevance. Without it, there will be less interest which could further lead to less usage as time goes by, in my opinion. It's one reason why I believe that Meiko, Kaito and Gakupo don't have as much content as they used to during V2~V3 era. Luka is following them on this example. Maybe slower, maybe as fast.
It's just sad to see things, that used to be more popular, slowly decline in relevance. I try to be okay with it, if that happens now or in the future.
Sorry for the long rant haha
Edit: Maybe I'm too dramatic and pessimistic...