The question you should ask is why there is Max altitude when it is matter of time ten minutes.
Indeed. It would be difficult to apply 2 limitations to the same thing. Surely, from an engine/performance/paid for approval point of view time must be the governing factor.
TOGA and FLEX are both take off thrust settings! Need to set MCT to remove the time limitation.
What if MCT is higher than the reduced thrust/flex setting? Does that make sense, then? Not being an AB man.