The rise in temperature is connected to the assumed, and therefore there is a performance margin. If it turned out to be a performance limiting takeoff, the question would not be asked, it would be visible as the temperature would rise above the assumed temperature. With a different QNH this is not that visible. The 10 is an arbitrary number. You cannot simply ignore.
I understand and agree on your reasoning in performance cases with large assumed temperatures. But there are also many takeoffs done close to the limit performance. And basic rules have to cover all situations.