He has a point - as I said in a vacuum outside of the realm of reality where there are additional trade-offs to be made vs. the ability to freely swap parts out without any form of validation that the parts are not malicious. As above, if you enable the end user to freely swap parts, it means any malicious party can also swap parts out either by intercepting your mail, evil maid attack, malicious eBay parts you may accidentally purchase, etc.
As I said above - if that's what you want, go nuts, there is plenty of open hardware, where this is possible. But you need to be aware of the tradeoffs. If those don't matter for your specific application (for example my gaming PC: could not give a crap) fair enough.
But for machines that I have personal data on and want to be secure, this additional security is a trade-off against parts replacement that I am willing to make; and for many users whether they realise it (the mechanics of how it happens) or not, the end result (more resistance to malware, hardware hacking, etc.) is a selling point for plenty of end users. It's a key part of how Apple keeps things like the iPhone, iPad and now, more recent Macs secure.