I agree with all except for the last statement. I suspect the Pixel will be Google's vehicle to release new features/functions, exclusively to their personal platform, first each year. They'll then roll them out to their partners afterwards. But lets face it, considering it generally takes 3rd party manufacturers another 6 months to update their software regardless, it's not like they're really doing anything different than what we've seen for years. How many phone's out right now are running Nougat?
Yeah that's possible, but it seems like they would damage their reputation with the oems. Imagine if Microsoft left some windows features which only worked on the surface tablets. They are already experimenting with their own AI's and it wouldn't take much for them to switch. But even so I find it hard to imagine that Google would limit Assistant as an exclusive and risk losing all that information gathering and eyeballs on the millions of devices that are left out.
Now if it's tied to new OS releases (which I did mention) then that's understandable. But as it stands right now Google specifically has taken this away from Nexus devices which can have the latest Nougat. So as an issue it may depend on the specific oem. Maybe a smaller issue with Samsung who's updates may not be released for 8 months later, but maybe more of an issue for oneplus who may release sooner.
Lastly, contradicting both our points about updates being tied to an OS is that Google has worked hard to make it's services separate from the OS and most of them are released independently, and don't need oem or carrier approval. Of course it's possible Google will set down hardware requirements for a phone to run Assistant, but eventually the oems would comply. Either way I still see it as a bad move for Google.