It Can’t (or Shouldn’t) Be Done...
There is an old saying that the person who says something cannot be done should not distract the person doing it, and there’s a lot to be said for that attitude.
However, if they’re spending a long time working at it with no significant sign of progress, when they could be doing something more productive, then perhaps the first person has a point.
If they’re not only spending their own time and resources doing it, but using other people’s resources too, then the first person has even more of a point.
And finally, the person who says, not that something cannot be done, but that it should not be done, may have a very important point indeed.
And yes, I’m thinking of nuclear fusion particularly, but I can think of a few other examples too.