Let me put it on the line - LFR and Flexible raid sizes are the most important raiding experiences currently available in World of Warcraft, and the upcoming Mythic 20 person raid difficulty is an atavism, barely even an appendix, that only a vanishing few players will experience when it is current. It exists for a sense of achievement and prestige that only a few players really have the time for anymore, and every year, that group of players gets smaller.
The reason for this is simple - as Tom Chilton put it, the demographic is getting older over time. People like me who played for the raid game back in classic are older. They have jobs, kids, schedules that don't permit the kind of time investment hard modes currently demand, the kind Mythic will demand. And it's not that you can't do cutting edge raiding in, say, six hours a week. I'm not arguing that you will have to put in 20 hours a week to do Mythic. I'm arguing that even scheduling one or two nights a week and being there reliably is actually really hard when you have other commitments that can often demand your time on a moment's notice - in essence. being able to go when you want/need to raid instead of when the group is scheduled to go is a huge boon to that aging demographic. For all the elitism, all the sneering, and all the slurs directed at the LFR player base, the feature allows people who love raiding but who can no longer commit to scheduled WoW play a place to do it.
You can ask if this is healthy for the game as a whole - whether or not your answer is yes or no, though, there is no escaping this simple fact. WoW is a decade old. Many of us playing it have been here for years now. Even players who started in Wrath or Cataclysm have now been playing for years. This is an aging game with aging players, this is the reality of the situation. And this means that more adaptive raiding solutions are going to keep presenting themselves.