Part of the issue with old smartphones and batteries is that every battery is a snowflake so eventually nobody is willing to produce the replacement battery anymore.

Contrast this to car batteries where the form factor is specified within certain limits.

This is, unfortunately, a point where the law probably has to step in and say "battery replacements must be available for 20 years" which would make it unpalatable to be a snowflake anymore. At that point, the suppliers would get together and agree on a "standard" battery form factor.