I think having an avatar is vital to whether or not a synth will do well. Whenever a new voicebank comes out--no matter what engine it is--the first comments on it are about the design. It's what many people associate to the voice. Pretty much with any kind of synth plug-in, no matter what instrument it is, there's always a picture of the instrument to give the consumer an idea what it will sound like. With the human voice, it can go many different ways. With Miku, we see it's an anime girl's voice: cute, and sweet. Cute is basically the norm in the vocaloid fandom because that's what we've been used to since the early 2000's. It'd be kinda hard to market a look different from what we're all used to.
But I do wonder if having anime avatars representing synths is as important as it used to be. We've seen the rise of OC stories and PVs in the Vocaloid fandom, completely ditching the Vocaloid avatars. But then again, non-anime backed VBs still struggle to do well - even the standard V5 banks don't have much headway in the fandom from what I've seen, and it's partially because of the unappealing avatars for them. (I mean, people still use them and create fanart of them, but they're not exactly on the same level as the Cryptonloids, Gumi, IA, Flower, Una, etc.)