The problem is that the 15'' pays for the sins of the 13''. It has to be the same thinness and share most of the same components, because of economies of scale (which are basically Cook's religion). But the priority for the 13'' is to be "a better Air, at Pro prices", so most trade-offs made there (33% smaller batteries, no high-power Ram etc) end up crippling the 15''. The artificial push to merge Air and Pro lines sacrificed one half of the audience.

Apple should have taken the boring, predictable step of giving the MacBook a 13'' version, rehashing the 12'' if necessary to match; that would gave been the super-thin Air line. Then they could have designed MBPs around the 15'' and power users -- which is what they basically did to great success in 2010-2012. But I guess the current solution has better margins, so 15'' users can go screw themselves.