There are some large capacity LIPO batteries but not for a small devices.They just using a pack of 2 and more batteries to increase the capacity. This is what upcoming istick 50w will have. 2x2200mA batteries. The wiring with the non-replaceable LIPO is even worse because they have to solder them to the connectors instead of using a regular contacts like for removable batteries. And like you said some ppl don't want to use built-in USB charger and they take the battery out to charge with the external charger. With non replaceable batteries you can't do this.

Charging life cycle of the lipo batteries is 300-400 times. And with the capacity of the battery is diminishing with the each charging. So yes, average 6-8 months of a life spin.
Some "funny" video about Lipo batteries
https://www.youtube.com/watch?v=7zrXTkqkrWE
There are probably millions of them are fail over the years. They called mini ecig or disposable.And they keeps failed even today. This why ppl are switching to modes with the
replaceable batteries.