Isn’t it more likely that the quality of the voice assistant isn’t that important to most buyers?
The day Siri will work equal to/better than other competing systems, it will be used as a main reason for which you should buy an Apple device. The day before that, it will still be something that users actually don't want.
EDIT: and to be a bit more clear that I'm serious with this, it actually makes sense. If a feature doesn't work well, you find ways around it. And if you are used to an ecosystem, maybe you don't even know how well another ecosystem works. And finally, if that ecosystem works for you well enough - or that device - you probably really don't care about that missing feature.
Apple (and Amazon) get many things right, and actually seem to pay a tiny bit of attention to user needs. So it's really not worth playing with the google stuff because you end up in these weird nonsensical hells - everything is amazing, and then they just drop the ball in a key corner.
For example, my google work calendar is EASY to integrate with Alexa along with my personal calendar. Fantastic, what's my schedule today works great. Google - falls flat on its' face for this, despite being a paying customer of their Gsuite. Their approach is just full of excuses here, and ignores that this desired interaction works well on their COMPETITORS devices but not theirs.
Just one (of many) examples. They have some sort of eventually consistent backend more often for stuff so you also get weird states that you can't delete things, changes take longer to "flow through" etc.
My guess is that Apple management doesn't see voice assistants as the "next big thing" to differentiate themselves from the competition. I think VR/AR is where they are focusing and Siri is on life support.