It structurally pays less per GPU as you add more GPUs, because the security budget is a zero-sum budget. (though, that's only if you ignore extrinsic effects, and some people argue that more security = higher coin price = higher security budget).
Even if the security budget is zero-sum, the amount of actual security added to the network with each additional GPU is constant.
It seems like your argument is that cryptocurrency needs to use over half of the computing power on earth at any given time to consider it secure. That's a very bad standard. Visa and MasterCard don't need this much computing power for security.
Security is an easy argument to reach for when you want to justify something stupid.
