Thoughts on Miku singing in English:
Intelligibility is important to me, but there's also a point where you have to respect a vocal synth for being what it is. Miku English is a vocal synth of a Japanese woman singing in English, which isn't her first language. That's not something you complain about; it's part of the color and part of the character.
I personally also find her struggles in this area thematically apropos. All of her users are struggling with something, whether it's with doing the music thing or something else. The fact that English takes a bit more work for her fits well into that idea--it's like there's something she struggles with, too. That shared struggle has been part of her character since "Miku Miku no Shiite Ageru."
That said, I wouldn't be opposed to an AI Miku who could do English more smoothly, as long as it sounds like her. There was nothing wrong with the direction of V4, IMO, for example.
(Continued below.)
I wonder.... If Miku AI became a thing, and we're worried about it sounding like her, why not train the AI partly on her classic songs? Ask the big classic Miku producers-- Deco*27, kz, Mitchie M, etc.--if they'd be willing to have their 2000s-era songs used to help train Miku's AI. CFM's probably the only company with the necessary relationships with its fanbase to try something like this. Some of the producers might decline, but maybe a bunch of them wouldn't mind--maybe in exchange for modest one-time compensation, or a grateful thanks on the box. The songs are over 10 years old anyway, and producers' primary output is songs, not tuning (though it might be a somewhat bigger deal for Mitchie, who's known as a tuning god). (Or open things up and ask for volunteers to have their songs used.) Throw those songs from all those different classic era producers together, along with the human training material that would've been used anyway, into a blender and see what the AI comes out with. Shouldn't it be less AI-sameness and more of what Miku's iconic sound has come to be?
In that scenario, you have Miku and her fanbase teaching the next generation of Miku. What could be more appropriate than that?