Zero sum or not, the attitude you have about budget discussions is why we are about to print ourselves into a depression. If you can't discuss prioritizing spending based on whatever criteria applies to the situation, you simply can't discuss budget concerns.
I do not take the view that the budget our taxes pay for is beyond reproach. I have no problem evaluating it from the perspective of 'money is not infinite, and no specific agencies spending is so inherently special that it can not be discussed under the context of where potential waste or inefficient spending happened, and how it can be adjusted to avoid that in the future."
'X seems like a better use of the money and more representative of what the taxpaying public wants it spent on, to ensure things are prioritized properly while properly representing 'We The People' -- as such it may be worth looking back to determine if this money was allocated in a manner that provides the most benefit to the taxpayers while maintaining consistency with the agencies core responsibilities' -- there is nothing wrong with this type of discussion/analysis, and anyone trying to claim there is has there own agenda they want to impose on you.
Your line of thinking leaves absolutely no room to discuss the budget in a meaningful way, because 'having priorities' is not allowed under that framework.
It's a dialectical trap meant to make people fall in line. I'll pass, thanks.