There are a number of points in your question that are worth exploring.
Firstly, is it rational for neuroscientists to support a physicalist view? The answer is an emphatic yes. All the scientific evidence, so far established, leads to the view that consciousness is a property of our bodies. As such it is perfectly rational to hold a physicalist view.
Secondly, is it possible that future evidence will overturn that view? Absolutely it is possible. It would be wholly unscientific to hold a position that you wouldn't change in light of new evidence. For example, if, tomorrow, a gateway to the afterlife opened up and you could visit it and return, I imagine most, if not all, neuroscientists would amend their position.
Thirdly, because of the possibility of new evidence should we hold a position assuming that new evidence? That depends. If the expectation is that the evidence is highly likely to be true but we just don't have the ability to prove it, then assuming it is true is quite common in science. Most physicists, for example, were pretty convinced we'd find the Higgs boson as it was the only part of the Standard Model that hadn't been confirmed.
If, however, there is no good, scientific, reason to believe that a particular piece of new evidence will turn up then generally it doesn't make much sense to assume it will. The existence of an afterlife would currently fall into this bucket.
Note, the difference between these points. Just because it is reasonable to expect some new evidence to turn up, it is not necessarily reasonable to expect a particular piece of evidence to turn up unless there are other strong indicators for it.
This is why your 50/50 point is not valid. There is a much greater chance than 50% that we will get a greater understanding of consciousness in the future. But a much lower chance of us finding evidence for the afterlife as there has been no scientific indication so far.
As to your last point on wild swings of opinion. To be honest, there generally aren't that wild swings. Just some people have a strong view one way and others have a strong view the other way. I would not expect scientists to move away from a physicalist position unless there was new evidence that prompted it. Until such a time, a physicalist approach is the most rational even if it eventually turned out to be incorrect.