Most phones can have battery removed somewhat destructively, but without affecting the rest of the phone.

Generally, as long as you keep the phone plugged in, the battery should be safe virtually indefinitely - the battery management on board will keep it in a state where its a constant charge which means the chemistry will be stable.

There were several generalizations in that statement that align with my similar fears to the OP. Most firmware should minimize the charge cycling, most batteries should be stable at constant charge... most isn't great for something that I want to sit in the corner undisturbed for a decade just chugging along - I have a few old desktops I use whenever I need a stand alone server or to host something web-live for a while. They'll eventually have hardware failures, but I have a lot more confidence that when they fail it won't be dramatic or destructive - ditto with old laptops, the serviceability expectations are much higher than phones so I have yet to meet a laptop I can't pop open and just pull the battery out of to run on AC alone - in the case of a power failure the UPS can't cover I'd rather the machine just power off rather than needing to deal with the possibility of dramatic failure.

I think if you're considering re-harvesting old devices to use for hosting and get far enough down your list to get to phones then you've likely got enough constant maintenance costs in overseeing things that the additional worry of fire risk just isn't worth it.

> Generally, as long as you keep the phone plugged in, the battery should be safe virtually indefinitely

What is your source on this?

I've replaced the battery in always-plugged-in iPhone 3 times over 10 years because it was expanding into a spicy pillow.

I too want a way to run phones directly off of USB power, without a battery present.

Of the six old Android phones I have around, two of them I don't dare turn on due to swollen batteries. I guess it depends how old the devices are whether this was a real risk, but I won't leave devices plugged in anymore for this reason.

I'm not educated enough in this area to have any expertise, however, in my personal experience leaving a lithium-ion battery plugged all the time results in scary semi-exploded batteries that also stop working.

Would you say this is a chemistry/QA problem? Have there been advances in battery / controller technology that achieves the above?

Depends on your phone. Just has to replace the battery on a generally always-plugged in Moto (at least after a certain age). Battery had pillowed out. It's acting as our "landline" with a link2cell on some old DECT handsets.