Service life of Li-Ion goes down substantially based on amount of time sitting above 4.1xV. Unfortunately most charge to 4.2V for that bit extra runtime. Life based on depth of discharge is not so clear cut. I wouldn't try to make it a rule to discharge down to 20% then be in situations where you are running out of power so can't use the phone, unless you have a relatively high capacity battery and know its limits vs your use cases.
Just about any battery is replaceable if you have the tools, and for older folks, eyesight and steady hand to do it.