That's a worthy goal, but so are "A computer should not betray me if someone else is a user of the computer temporarily" and "I should have full freedom to modify a computer I own." And I think you can't satisfy all three at once.
You can satisfy "do not betray the user" and "do not betray the owner" if you have a locked-down computer like a Chromebook that only accepts signed firmware and signed OSes. That way, no user of the computer, owner or otherwise, can make permanent changes to it that will compromise other users; guests are protected from the owner just as much as the owner is protected from guests.
You can satisfy "do not betray the user" and "have full freedom" if you have a computer with no protections against OS modification (the traditional PC architecture), but that is susceptible to physical attacks, where a user can subvert the owner. And every user needs to subvert the machine afresh in order for the computer not to betray them to the previous user; this is a risky model for things like shared computer labs.
You can satisfy "do not betray the owner" and "have full freedom" with an architecture where the owner gets a password/key that unlocks management functions or BIOS reconfiguration, and physical access (short of desoldering) is insufficient to access those functions.
I'm a fan of that latter model. Of course, I do have the resources to own a computer of my own for my personal computing, which biases my preferences. There's a good argument for building a more just world for people whose only computers are shared computers (libraries, work-issued laptops, etc.), but I think smartphones are becoming ubiquitous enough that we can expect that everyone will soon have a device they own for their own computing.