and they still have all 6 members
True. (Although a member left during their 'Hush' era in 2012, but that was due to her own reasons of wanting to focus on something else).
Apink members have so much trust between each other that they have all said they want to be in Apink until they are old lol. Sure, each member have had solo work, but to them, they've always put Apink first.
And their staff has been with them since pre-debut. Their company started as a subsidiary in Cube Ent, but during a couple of transfers up to where they are now in Play M, their staff have gone with them.
Their trust/bond and perhaps even their staff have perhaps given all Apink need to succeed and last for as long as they have. They are to be admired and hopefully other groups strive to be long lasting like Apink.
Also, their music progression has brought in new fans for sure. Since 'I'm So Sick', I know newer K-pop fans started following Apink. So their management is doing all the right things to keep Apink musically relevant, for both old fans and new :D.