I'm sure this has been said but mythic is not a new thing at all. Let's try this way:
MYTHIC IS HEROIC OF THE PAST
The heroic difficulty you want back IS mythic. The best part is that heroic difficulty was a fixed raid size as well, just a different one at 25 player. Arguably it was 25 OR 10 during Cataclysm and MoP but those are still just two different fixed sizes, furthermore the WoW community was constantly bickering at one another about which is harder. Flexible raids are good for what is called "Normal" now which was originally the "Flexible" difficulty below normal. They got EXPANDED to what is called "Heroic" now, but is actually what you've called "Normal" in your post.