I don’t really see why people are against it, personally I never buy locked devices but they are usually a chunk cheaper and there is always an option for a locked device.
If telecoms were making certain phones exclusively locked (as in not selling unlocked phones) it would be a problem. But rn it seems that it is an easy way to save money if you like a carrier.
Boots theory doesn’t really apply because it is the exact same phone/hardware. Plus most people don’t really care about reflashing their phone.
As for the privacy stuff I don’t really know much about it in the context of locked phones so I’ll take your word for it.