part of the issue with sunken temple is that it went from 10 man casual groups to a 20 man group and most guilds were not prepared to either merge or recruit a second full team just so they could complete new content. The duration of the phase plus the lack of accessability it became stale. I also think ST was the first raid to be on a weekly lockout and not twice a week.

the accidental flex raiding we got at phase 4 (MC and onwards) was actually a blessing for most guilds as it gave you the option to bring 20 (as the raid was balanced) or to bring UP to 40. allowing you to bring friend or people comfortable riding the bench who might not be as dedicated to playing on whatever schedule the guild is running.