I have no idea, but just speculating on your situation. Yes, you may be Plan B, but I think there may be other possibilities.
Assuming that he is acting in his best interest, he sees his best interest as being separated from you and not divorced from you. Why might that be?
*Does he has a religious/moral/social obligation/belief system that prevents him from getting divorced? He can think of himself as a good man for remaining in this state (generously keeping you on his insurance, etc.)?
*Does he still love you/have feelings for you/feel married to you...but is so angry about issues/hopeless that they can't change that he can't stand co-habitating with you?
*Did you have sexual incompatibility? Does he feel emotionally connected to you (wants to talk to you every day) but not sexually?
*Is he waiting for something, like the last kid leaving the nest? Or finding an OW that he wants to marry (something more serious)?
*Is he in the middle of a multi-year MLC fog where he is trying out different things (freedom, independence, moving out of the house, dating other people), but is not so certain about wanting a D?
Gordie 40s W 40s M20+ kids 2016 BD W fantasy affair w OM1 I do everything wrong 2017 I start to DB W says TLTL files for D PA w OM2 2018 I do LRT W drops filing and OM2 situation slowly improving