Comcast has 0 control over what runs on my modem (Spectrum in my case). As long as the modem is DOCSIS compliant, it will work.
The same applies to unlocked phones. The service provider has 0 control over what I am running on that phone, and they don't control the updates (the OEM does), but as long as the baseband firmware complies with established standards, the phone will work. This was mandated by law some years back in the US and I am certain it's been the case in the EU for longer.
What you seem to be referring to is telco customized phones (subsidized ones), and in those cases you'd be correct.
The same applies to unlocked phones. The service provider has 0 control over what I am running on that phone, and they don't control the updates (the OEM does), but as long as the baseband firmware complies with established standards, the phone will work. This was mandated by law some years back in the US and I am certain it's been the case in the EU for longer.
What you seem to be referring to is telco customized phones (subsidized ones), and in those cases you'd be correct.