Batteries only realistically have a few years of optimum performance, if that.
Remember they are charged EVERY time the bike is used, that's a lot of charging and discharging.
A phone's battery has a fair life expectancy of TWO years so once you consider the shite a bikes battery gets put through, especially with hot/cold temperatures, then it only makes sense to change it when you get a hint of problems.
And for the amount of years you'll get out of a decent brand, it makes no sense to cut corners and save £10 or £20 on an inferior battery.
The chances are you'll only ever buy one (perhaps two at a stretch) battery for the whole time you own the bike.