Obviously, the societal use of resources on patients with severe TBI is a contentious issue.
I think a starting point in the discussion is determining what the likelihood of recovery is. I cannot be sure, but reading into your question, it sounds like you are working under the assumption that most patients with severe TBI do not have a good recovery. I don't think that assumption is true
The extent of recovery from even severe TBI is highly variable, and often a good prognosis cannot be made for quite a while after the initial injury.
Second, we are dealing with a moving target. There are new techniques in the management of severe TBI (some are new because they are newly discovered, some are new because they were known but not widely used) that are likely to greatly increase the extent of recovery, including cooling at the time of injury, or the protective use of hormones such as progesterone.
A big issue is that much of what we call severe TBI is a heterogeneous condition, with remarkably different rates of recovery. This is why the Terri Schiavo case was so contentious- the two sides were not in agreement in the extent of her TBI.
So that is the long winded framework for my answer.
In the big picture, it is not clear to me that society should be making a financial committment for more than the base services of healthcare. I think that the care of patients with severe TBI should be considered in the same pool of dialysis in the elderly, ICU management of CHF in the terminally ill, NICU care for premature babies, etc. I am not against the medical care of patients with severe TBI, and I would want it for my loved one, but it is not clear to me that the US government should be paying for it. But at the same time, I am not sure that the US government should be paying for infertility treatments or ICU treatment for a baby born at 29 weeks. In all of these cases, I would want medical care for my loved one if I was in that situation, but that is putting my personal needs ahead of societal needs.
But, if government is going to be paying for these comprehensive medical services, I don't see a reason for severe TBI to be excluded, since the outcomes are not necessarily any worse than it is for these other non-basic health care needs.