I am a big fan of removable batteries but even I can see the huge advantage of USB-C Powerbanks. An additional ThinkPad battery can cost ~$100 iirc. And this battery will only ever work with this specific laptop model.

A powerbank for the same price will probably have a higher capacity, and be able to charge any USB-C device.

All this is not a reason for integrated batteries, but Powerbanks using a universal interface are clearly more practical.