Kind of, but doesn't that assume a linear utility function for holding eth which I think seems unrealistic? Plus, there are costs associated with holding eth (cold storage etc) which seem fixed, so its cheaper to securely stake 1 additional eth when you're already staking 100eth than when you're not running any. Plus fixed costs of actually running a staking business (by analogy with a mining business).
It’s the same price to add 1 eth when you own 100 eth as when you own 1 eth