I said smaller battery and connecters allowed for thinner phones.

Just making the batteries non replaceable makes them able to be thinner, and they always had that ability. They also had the ability to put in smaller batteries if everyone was clamoring for super thin phones. They just weren't.

And mass production is cheaper than low production.

True

So if all phones are designed the same, then the factories can be tooled all the same accross all models.

Uh, no. That would only be true if all models of phone from the same factory were made exactly the same way with the exact same machines. That's not the case. They make different models of phone. They could easily make one model with a user replaceable battery and a different model without one.

But you said they exist… it should be easy. Unless… you were talking out of your ass again.

They do exist, and it was super easy. They still make Ford Rangers. Try google, it's the first thing that pops up when you type in New Ford Ranger. I guess you were talking out your ass again.