Sure, that's why Vanilla needed so little time and was so casual friendly, that casual-friendly style paid for the future expensions.
That's probably the reason why 20% of guilds usually kills the first mythic boss and around 60% of the guilds kill the first boss in heroic. (hint : it's more than 5% of the playerbase)
Probably the reason the devs said that and made flex :
Besides, Raid Finder was tuned for randomly matched groups and generally lacked sufficient challenge for even the most casual of organized raid groups.