In the perfect world Mythic would be flex as well. But that perfect world doesn't exist. Blizzard was unable to balance the hardests content around around various raid sizes. They've tried for 2 expantions and each attempt to fix the issues existing there created 5 more issues. 20man Mythic isn't the perfect solution. But it's a working compromise. Yes, it wasn't supposed to satisfy everyone. It was supposed to create a stable foundation of hardcore raiding that works.
Unfortunately, the coexistance of 10 and 25 man for the hardest content ment the slow and painfull death of 25man guilds. Why you might ask? Before people are lazy and will ALWAYS chose the easier route. And 10 mans were the easier route for obtaining the best rewards. You think there weren't people stuck in 10man raiding because all of the 25man guilds on their server died off? You see, people who want to raid in bigger raids don't really have a potential to lead a bigger guild.
There was ofc one more issue. Lack of entry raiding content in Cata basicly stopped the flow of new raiders. That was the reason many guilds turned into 10mans back then. Cata was a 10man or die expantion. Only the best guilds survived that period because they still were able to live for few more month canibalizing dying 25mans below them. Imo, if flex didn't happen, any raiding above 10man would be gone by now. Flex was a breath of freshness into quite messed up raiding scene.
But as I stated in the begining, top difficulty cannot be optimized around various raid sizes because the challenges switch depending on the raid size. If Mythic were flex I can assure you, people would base their tactics around the most optimal amount of people a cheese the encounters to the maximum.