This. This, this, this, this!

My linux computers are rooted. I can get root any time i need it and nobody is refusing to offer their sevices on linux because it is vulnerable.

Nobody ever points out that when any app wants root, you get a dialog to ask if it can have it. If you don't know why it's asking, say no. It ain't rocket science.

Now, if you are going through customs and you don't want them to copy your phone and read all your personal documents, that is a different situation. Lock your bootloader unrooted and encryped to the nines. Preferably use a phone with almost nothing on it.

Pre-locked bootloader times ive had multiple android devices be passed to me that were malware infected that changed the rom in a way that even a factory reset would not remove the malware. Locked bootloaders made it so the rom needed to be signed and unaltered on boot, fixing this. Root access also means apps can use and access api's in android that it normally cant, changing settings and things inside android it shouldnt. What do you think happens when malware comes in? :p

Imo, i agree what you said. bootloaders should remain locked but you should be able to somehow, in the bootloader, be able to add the os' signature/keys to the bootloader's trusted stuff like how secure boot on a pc keeps os signing keys and verification stuff inside the tpm.

This way you can install lineage os for example, tell bootloader to trust it, and lock bootloader again so nothing can be changed anymore.
I wouldnt take this from user input, as that is controlable by malware, but rather come from the OS itself. Maybe even during installation, idk