"Yeah but"s are actually essential to a successful project. The devil is in the timing. If you're still working around with the direction the New Thing is going to take, you end up with scope-jacking. If you hold it off until you're at the operational planning stage, you're golden. That's when you call it "risk-management".
There are always exceptions - usually when you run into legal issues that prevent directions you can take. Or invisible boundaries when you find out there's a duplicate somewhere.
But as a general rule, do whatever it takes to dodge scope-jacking.
