Chain running 14 +1 is not effort, especially as a well geared character, it's just an inane time sink. All in all, they system conceptually works BUT we (as you said) don't know how big the X is and if the numbers are final. If anything, i'll bring those 15 runs to at max 10, or even 8. As filling up the 3 slots of raiding is pretty easy sapmming lower difficulties, i'd add to them the same rule in place for M+
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Pretty confident it's something like that and i'm pretty sure the 1/5/15 will be lowered aswell. As said above, the major issue i see is not how the rewards are given but how much of a difference is there between the three type of contents.