Here's the first problem with the person who made the original post. There's no way they polled all of Area 52 and got that percentage. What they did was maybe polled a select few who fit the agenda they wanted. In fact, I play on Area 52 and that's the first time I've every heard of any "poll".
Using percentages to try and persuade your goal doesn't do anything since most of the time percentages are either heavily skewed or just made up.
As for 10 man's you can raid as a 10 man group right now. Evidently there aren't that many who are doing it or Blizzard would've said "Hey, let's add that to a mythic rating too". Instead Blizzard chose 20 man so that way people aren't excluded from raids and stuff if they are FOTM as it was back during Cata and MoP.
Overall though, 10 mans are still in the game and pretty viable. So not sure why the person you quoted thinks they need to come back when they never left. In case you, or anyone else didn't know, all raids flex between 10-30. So you can easily do 10 if you want or go over that if you want to invite more.