A body is the only way to convince the scientists who won't bother to take a closer look for themselves, and who completely and willfully ignore the opinions of those who have (including other scientists). The case for sasquatch is already overwhelmingly obvious.
It is therefore a failure on their part not to look, not a failure on our part not to produce, but their failure is no different than the failure of laymen: it stems from an inability of the imagination to grasp the concept that such a creature could simply be so elusive. It is a totally unique phenomenon, we have no other basis for comparison, and people just can't accept that another living creature could be that much better than us at something (at least without the use of technology - which is why people seem to have an easier time accepting the possibility of aliens than bigfoot).
Sasquatch will be proven just by us telling people it's worth it to look into it.