Estimate when to replace your phone.
Phones are retired for two reasons, and neither is speed. Battery capacity below about eighty percent makes a day's use impractical, and the end of security updates makes the device unsafe for banking and email. Whichever comes first sets the replacement date, and cost per month of ownership so far is the figure that shows whether keeping it another year is good value.
Battery runway
Battery months left = (battery health - 80) x 3, reaching zero at 80 percent health
Useful life
Useful months left = min(security update months remaining, battery months left)
It is the level manufacturers use to define a worn battery and where most users notice they can no longer finish a day. A battery replacement resets it far more cheaply than a new phone.
For a device holding email, banking and authentication apps, yes — unpatched vulnerabilities accumulate. Repurposing it for offline use is safer than continuing to use it as a primary phone.