In the days of DOS, the BIOS would load the first sector of a floppy disk or hard drive into memory and transfer control to it. You could replace that sector, and not a single line of code that you didn't write yourself would be executed after that. THIS is how it should be on a PERSONAL computer. A virus could do the same, of course, and gain full control of the machine, but that is not an argument against giving control to the user. You could blame DOS for not intercepting attempts to rewrite the boot sector and asking the user for consent, but completely preventing something like this in the name of security would be even worse!

Imagine if back then, there was a "secure boot" mechanism so that only a boot sector with Microsoft's cryptographic signature was allowed, and that boot sector code would in turn verify everything loaded afterwards. Linux wouldn't exist. So, neither would Android, but the situation would be much the same as it is now, only with Microsoft Phone OS instead -- and absolutely no concept of how it could be different, of a computing device controlled by its user instead of a giant corporation.