>If Apple starts spearheading changes in how phone modems work, could we imagine mobile towers playing along and converging? Or is it more entrenched than that?

Do You mean Telco Equipment vendor converging? Well first thing is that 4g / 5G or 3GPP is an open standard so anyone could implement it. Second is that there are only a few Telco Equipment vendor left already. There will still be insane amount of testing required to be done even if everyone were to use the same equipment. The amount of variables such as spectrum, regulations requirements, physical space and density as well as weather difference.