Yeah, i agree. Zero-G said it themselves that Sonika was an "experiment" lmao.
Like i said, it takes a specific niche and a good producer to make Sonika sound good. But for the most part, she unfortunately doesn't because she's just that difficult to work with. Though i can understand the appeal of trying to use her.
-Despite her low quality, she does have a specific vocal type and nuance that other english Vocaloids don't have, a British, soft, cheerful female vocal. (though you could argue Gumi English is similar enough to pass for a Sonika except done well, but i digress, and i dont know how easy she is to use, i hear shes got her own bugs too)
-Bragging rights for getting a LQ vocaloid to sound good. (i empathize with this more, as someone who enjoys pain and suffering)
Sometimes i can't help but imagine an alternate timeline where Zero-G didn't fuck with the vocal samples and kept them natural. I really wonder how much of a difference it would have made in her quality.
And on the topic of Vocaloid, you're right. It was always INTENDED as a replacement for human vocals, but it has never been capable of replacing them in its entire lifespan. Yamaha always had the goal of replacing human vocals, but it always marketed Vocaloid as a software that would "replace session singers forever" (lmao) when it was never capable of doing that. And to be honest, if it weren't for the otaku side of the fandom preferring the artificial sound of the software alongside the moe characterizations we get as compensation, the program would have been canned a long time ago.
Now this is gonna sound mean, but when normal people listen to Vocaloid, usually their first reaction is to feel a little creeped out. Thats not by chance either. Vocaloid exists right smack dab in the middle of the uncanny valley, and unless they're tuned to sound a little more artificial (with better expression), or in the hands of a REALLY GOOD tuner who can actually make them sound realistic through whatever black magic they use, they just dont sound appealing to people who aren't "used" to how Vocaloid sounds.
I suspect this is also another reason why JPloids are more popular with Western fans and producers, because its easier to excuse the un-natural sound of the voice when its in a language you don't understand. (that and also the synthesis engine just works better for the Japanese language)