iPhone battery replacement

The battery in SWMBO's iPhone has swelled.  Not surprising as it
is ANCIENT (6s).
She is asking for it to be replaced:  "I *like* that phone!"
Is this actually worth the effort, given that batteries are likely
knock-offs (dubious quality) and disassembly/assembly is undoubtedly
a lesson in fumble-fingers?  And, what's the likely life expectancy
of a "new" battery?
I've offered her either of two phones that I use but "they're too big".