Scanners cost a lot: ~4M for a 3T (the current 'standard') and much more for higher field strengths. We just got a 7T and, with all the trimmings (scanner, console, coils, the shielded room to hold it all, training, etc), it's in the $10-15M range.

As a result, time on these things is already in pretty high demand, even at facilities that are used only for research. The projects could be reprioritized, of course, but most of the on-going research is much less speculative than "someday, this can be training data for something."

It's not even clear to me that old MRI data will be particularly worthwhile. Photographic plates were, until recently, about as good as digital sensors. New MRIs are much, much better than old ones, and the tech will only improve more. Longitudinal data is useful in both fields, but for astronomy, there's no way to rewind time, so old plates are the only possible source for events from the past. We get new old people, however, every day!