Why can't computers have physical switches that enable/disable writing the memory that this piece of software is located in?
In this case, the RSA sig is still better, though. Imagine that anyone sitting between the Intel plant and your local computer parts supplier could flip the switch.
It's a weak argument, I admit. There should be both protections at the same time.
Asked from my N900, the only one I am aware of.
For example, the NSA intercepted deliveries of switches and installed their malware on them. Would be easy if there is a physical switch. Not so easy if you need signed firmware (I'm sure the NSA could still do it, but it would definitely be harder).
I'd argue that not being able to modify software subjects you to a higher risk because you wont be able to fix security vulnerabilities yourself.
Software and hardware outlive the companies that produce them.