What are you really getting from ESXi that you cannot get from a homebrew of the same? Sure there is something to be said about being able to move guests from one host to another so that the host can be maintained. There is also this exponential cost in order to insure that every single transcation is accounted for. And then you have to ask exactly what disasters are you preventing?
The counter argument to ESXi is any OS that can run any cirtualized OS as a guest. We typically do not deploy the same hardware resources to a bare OS that we would to ESXi but why not? Furthermore, it really depends on what the critical failures really are. If it’s HDD, SSD, power supplies, power, networking or bugs in the individual guest OS then there is no reason that homegrown host/guest could not do the job… just add some chaos monkey testing and there ya go.