An aspect of it that I've been taught is, rather than explaining why something can't or shouldn't be done, explain what it will take to get something done. E.g.

An aspect of it that I've been taught is, rather than explaining why something can't or shouldn't be done, explain what it will take to get something done. E.g. don't say "we can't do X because it's too expensive, requires too many people, is too risky, ..." Instead say "doing X would require raising additional funding or diverting funding, we'd need to pull people from other teams and/or hire additional headcount, will require some risk reduction investigation for two weeks before we can give a complete assessment, etc." Then let your CEO, board, whatever decide or help decide if the costs are too high.