So why do I feel your reasoning behind removing 10m Mythic to improve 20m Mythic doesn't make any sense? Because you can at any time make 20m Mythic that complex "epic" while still creating content for 10m Mythic. Look at Naxx 10/25, those fights were changed to allow smaller groups the ability to do the fight, sure the fight is easier in 10m but who cares the challenging 10m content is still there. I think the ENTIRE 10m Heroic community would be totally fine if the fight was changed slightly and even made "easier" so 10m can still do the content without being forced to raid 20m to be challenged.
Instructor Raz had pillars to MC, the spider chick you had to kill the guys beside her to remove the buff etc. Ya those fights were technically easier in 10m but you can make it so 10m achievement's are seperate we don't care, give 100% warforged to 20m and 1% warforged to 10m because their content is harder. Say that 10m Mythic will be easier then 25 Mythic and avoid the silly 10v20 debate that h10m raiders could care less about but don't remove content that has existed and players have played for almost 5 years now.
Removing challenging 10m content after 4 years and 3 expansions is one of the worst choices you have ever made. Seperate 10m Mythic and 20m Mythic, say they are different and 10 will be easier and be done with, if it can't be balanced so be it but telling us you want one to be better so were removing the other is a cop out.