An unpopular idea isn't necessarily a brave truth. Plenty of claims sit outside respectable opinion because the evidence against them is good. Being dismissed by experts doesn't make someone Galileo.
But the reputation of an idea and its truth aren't the same thing. That distinction matters even when most of the people invoking it are wrong.
Consensus can take time to change
The histories of handwashing in medicine and continental drift are reminders that an important argument can encounter resistance before gaining acceptance. Those histories are more complicated than a lone genius facing fools, and they don't vindicate every modern outsider. They do show why the standing of a claim isn't enough to settle it.
A professional consensus may reflect a great deal of accumulated evidence. It can also be slow to respond to new evidence, especially when changing it means admitting that influential people were mistaken.
The social cost of questioning a view affects who will question it publicly. That gives us a reason to distinguish confidence in the evidence from confidence that respectable people currently agree.
The same standard for an outsider
None of this makes a consensus worthless. Most of us have to rely on expert judgement in subjects we can't investigate ourselves. Nor does it mean every dissenting argument deserves equal attention.
It means the dissenter still has to produce evidence and explain how it supports the claim. So does someone defending an established view. "Everyone serious agrees" can tell us something useful about where to start. It can't substitute indefinitely for an answer to a specific objection.
The difficult position is to take expertise seriously while remaining willing to examine a challenge to it. Automatic deference and automatic suspicion both spare us that work.
I want to know what would establish that the accepted account is wrong, and whether anyone has actually supplied it. The outsider's reputation doesn't answer that. Neither does the reputation of the people who dismiss them.