One thing that I've encountered on other sites is that there's sometimes people get attached to an idea, and then any attempt to quantify that idea that comes out below what the proposer "wants" or points out how much is still unknown in the context of that idea, is perceived as an attack to be obliterated. But given that we're likely to have lots of alternative ideas for a given problem and need to prioritise, I think quantifying things is very important.

So I would modify things a little: it's ok to try and (with honest intent) quantify some aspect of an idea, or clarify what aspects of a proposal are "understood" and which bits are "research projects", without needing an alternative idea yourself. Beyond that, I think your idea is a good principle.