apart from that the form factor is what its about.. shaving the odd millimeter of these phones phablets and laptops seems a major selling point.. having a (user) replaceable battery isnt..
That seems to be very important selling point, but I think that the most important reason for manufacturers is cost reduction and thus bigger profit margin. It's not that they are going to lower consumer prices because of the "non-replaceable" batteries.
Many people still don't get it that phones thickness is not so relevant, unlike its length and width. In other words, marketing brainwashed people about phone thickness. It doesn't make any difference if a phone is 10 mm thick or 6 mm thick when its dimensions surpass 140 mm in length and 70 mm in width. This is because 140 mm length 70 mm width are pretty much the maximum for "comfortable" handling for an average person.
A few years ago researchers concluded that 4.3" smartphones are the best compromise between comfortable handling and usability. Today, we have 5.0" phones which are not very comfortable for use and 5.5" phablets which are way too big to be comfortable. They even don't fit comfortably in the pockets and there's no way that you can use them with one hand, unless you are 2 meter (6'7") giant.
i always think 300 to 500 charge/discharge cycles as the life time of a lithium battery.. many a phone in daily use will hit these figure easily within two years.. oddly enough when these phones do fail its now called a battery fault.. he he
things that dont get used as much will have a reasonable life span before the battery wears out so will some phones but in the end it does come down to how much use a device gets..
There are many ways to prolong battery life, but hardly anyone uses them. For the example, my battery lasts approx. 5 days before I need to recharge. People look at me strangely when I show them my battery statistics.
First, turn off automatic brightness and set it to manual to about 40-50%. Mine is at ~40%. Turn off GPS, turn off wifi when you don't need it, turn off automatic synchronization and synchronize manually only when you need it. ...and there you go! Now battery will last for 3+ days without charging.
one other thing that has not been mentioned is the fact that these batteries simply degrade with age.. the higher level of charge they are kept at the quicker they degrade.. how quickly isnt exactly known.. at least not by me but degrade they do..
Batteries tend to produce extra heat when charging and it affects their lifetime; apart from charging/discharging cycles which slowly but surely reduce their lifetime.