I'm pressed for time right now so I'll make this quick. A few points; The body IS a life support machine for the brain, to make an artificial one would require you to replicate the function of almost every single organ and tissue type (far from what we can do now). There are huge interactions important for health and homeostasis beyond pumping blood, just off of the top of my head you're going to have to solve all the problems of hormone regulation and feedback e.g. interactions between the pituitary gland and the adrenal medulla. So in summary: The technology needed to keep a head a live separate to a body is science fiction, we don't even have to knowledge to be able to tell how we would go about that let alone actually doing it. You'd also have to solve the problem of building life support machines of such intricacy that they can run for decades on end with zero failure.
Secondly if we did have this machine what benefit would it do to the patient to keep them in what is essentially a permanently paralysed state? Sounds like my idea of torture if I would have to be rigged up like that for decades on end with no hope of a cure.