Should I be surprised that you two have no valid rebuttal to these basic facts?
Should i be surprised that you are completely unable to get a very obvious joke?
But if you want a rebuttal, fine, here's a flowchart.
X starts by saying that 15 for all is totes awesome
Y comes and sez 15 is totes ridic. 12 is mucho awesome tho, because a collective set of stuff that we'll call *safe*
X then goes "hmm, safe is a good point" and then goes balls deep with safe and settles for 10, which is still an increase, but saferer.
Y can then agree with X or try to defend 12. If he agrees, the discussion ends here (unless X decides to be a bigger dick and push for 9, since saferest). If he defends 12, then X is very likely to just straight lift the argument for danger 12 and apply to danger 15, because why the hell not.
Repeat until Y comes up with some very specific data to support his very specific magic safe number.