You can't compare to a raid that was out for 14 months; and had boss health dropped by 60% in the prepatch.
On top of that. Siege had 8-10 very easy bosses and 4 hard bosses. The difficulty scaling in it was ridiculous. BRF is much more linear in comparison.
That's nothing to do with the mythic model of 20 that's about a raid tier being balanced.
On top of that. Most guilds being 10 man = more guilds clearing a raid. Obviously. If 100 guilds kill BH mythic with a 20 man roster and 200 guilds had killed Garrosh at the same point in time with a 10 man roster. There's no difference, it's the same amount of players. 25 mans were still a minority in 5.4 even with HC WF gear. Most guilds scaled up after they finished the raid.
A thing to note about this is that subscription numbers had a greater influx at the start of WoD than pretty much any expansion ever. In the last couple of expansions about as many people as re-sub will quit again before the second tier, so the fall off feels a lot greater this time.
We've had our share of roster issues, but we also have no shortage of applicants having been able to maintain 25-30 people on the roster for the entire period of WoD so far. Intelligent and active recruiting nets much better results and most guilds lack one or the other, which is where their problem stems from.
Ignorance can be fixed, stupid is forever
Twitter: @erlaya_warrior
Guild: Invoke
Recruitment: Need Disc Priest, Demon Hunter & Any other awesome DPS
Mythic Progress: 6/8 Uldir & 5/9 BoD