Need a more tactful argument? Scott Sehlhorst’s article Code Debt: Neither a Borrower… is a great discussion on the cost of sacrificing quality in a development effort; the lack of quality is a debt you will eventually have to pay back. Although the article focuses on pressure to deliver code without quality, the points apply to requirements as well.
Scott summarizes with a great quote from Mishkin Berteig’s article Technical Debt: “In other words, every time someone asks a team to let quality slide, they are asking the team (and the organization) to take on debt with an unknown interest rate. Which is lunacy.”