.. in support of mrjet ..
(a) launch with a KNOWN defect requires that the MEL considers the intent of the certification standard and come up with appropriate restrictions to achieve a similar capability with respect to risk.
(b) in-flight failure is a crew-assessed situation .. if you don't have any option for a pack (fuel/diversion) then the risk of normal level is accepted .. but, if you have enough fuel, then it would be difficult to argue the decision to go above 250 at the enquiry ...
(c) in general, an in-flight failure warrants review of the MEL to get the guidance therein .. and, if you reasonably can do so .. it is a good idea to observe the MEL restrictions. Keep in mind that there often are requirements which the pilot may not be aware of ...
(d) it is not the OEM imposing the restriction .. the MEL is based on the MMEL which is approved (and issued) by the FAA.