Isn't x64 a standard? Couldn't another company create their own implementation of it and design their own processors for the paranoid to use?
The problems are economies of scale and the perfect being the enemy of the good. What is the interest in an expensive custom chip when you still can't trust that it wasn't compromised by the fab?
With testing amplification, you can then set an upper bound on the probability that the chip will run a backdoor.
You're only locked in if your personal requirements mandate the smallest lithography process and the highest clock speed.
Where does one find a MIPS or ARM laptop? Who manufactures them?
Also, the Microsoft Surface RT tablets were ARM and fairly impressive from a hardware standpoint (sturdy, nice screens, etc.) You should be able to get a used one on Swappa fairly cheep. I know there was some credential leak recently, but I'm not sure if that's gone far enough to make it a useful development machine. (Also, it's a tablet with a kickstand and a detachable keyboard...)
I'm pretty sure POWER still fulfills requirements for "highest clock speed", not x86/x64. That might be based on outdated information, though; I just recall POWER being the first to hit 5GHz CPU clock speeds even in spite of IBM's lithographic processes lagging behind Intel's.
Even if it was, being a standard does not mean that it is free, easy and legal for others to implement. Some standard bodies require a patent sharing agreement or licenses being provided for a "reasonable" cost, but what is reasonable is often a topic of discussion.
x86_64 also incorporates a 32-bit compatibility, which is copyright protected by Intel and only a handful of licensees exist, with no new licenses having been signed in the past decade or so. I don't think you can implement AMD64 without also acquiring an x86 license from Intel.
x86_64 is an AMD standard, I believe still under patent.
Intel and AMD cross-license these patents to each other. As a result, any other company wanting to make x86 processors would have to purchase rights from both companies (assuming they're willing to, which is a big if), which would instantly make their products cost-uncompetitive.
x86 is also copyrighted, but it's expired. Most the parents have expired and it's starting to become more common to 486 and 386's in very cheap embedded controllers.