It’s my own device so I should be allowed to let the manufacturer make it secure so I don’t need to worry about security.
I don’t want _all_ my devices to behave like that but I definitely want my phone to be more trustworthy for banking and government service purposes.
That's not the problem at all.
The problem is that manufacturers are forcing everyone into this scheme for the express purpose of mass surveillance and control.
It has nothing to do with making your device "secure"
> I should be allowed to let the manufacturer make it secure so I don’t need to worry about security.
You can still do this by simply not rooting your phone, or replacing the manufacturer's cryptographic key with your own, or altering whatever other 'security' measures are in place. What you're asking for is to have no other choice but to give the manufacturer control over your devices.