Is a plug-in long term sustainable?

Look to the longevity of Prius cars, including the Prius Prime.

They're essentially the same power plant in the Rav4 prime.

https://www.torquenews.com/6626/toyota-hybrid-owners-speak-out-battery-length-and-cost

 

your battery is never at 0% or 100%.
there is a buffer at both ends. the size of the buffer depends on the manufacturer's choice, how they programmed the BMS.
At least in canada, the electrical powertrain is warrantied for 8 years, so you should not see any significant battery degradation in that time frame.

I don't think you can compare the battery life in a PHEV to a pure hybrid. In a pure hybrid the computer decides how much to charge and how much to discharge the battery and it will likely keep it close to the middle of the charge range. in a PHEV you can choose to "fully" charge and discharge the battery every day. But again, the manufacturer could choose to have a larger buffer in a PHEV than in a pure hybrid.

there are many factors that determine the battery life or degradation. I'd be much more concerned about the temperature management of the battery. Cars with actively cooled batteries will last much longer than if they have passive cooling like the Nissan Leaf.

 

I'm with Electric Eddy, I'm just not worried about it. The warranty on a Toyota battery is very long and I trust the computer to know what to do, if it doesn't, then it'll be covered under warranty.

I come home, I plug in, and the battery is "full" every time I leave the house, but as already mentioned above, you're never really charging to 100% anyway, there's a buffer. I don't see any reason to add a 2nd buffer - I doubt I know what is best for the battery any better than the computer that controls it does. If my assumption is wrong... I have 10 years of warranty coverage. I don't know what a new 18 kW battery pack might cost in 10 years but I'm willing to bet it's less than it costs today, would probably work better, weigh less, and charge faster. But I also know there's very little chance I'll still have this car in 10 years anyway - with the advances in EVs and solid state batteries I know I'll never make it that long before plunging back in for a full EV lol. Heck, the Ionic 5 already has my head turning