I think that basically, (at least as the male Lutece sees it, or happy ending-wise) since Liz can see every door ever in existence, she's basically 'cheating' as we humans see it, and reaching the perfect decision/time/space through the tears that will have the desired effect on the multiverse to erase the Comstock decision from the timeline forever.
I'm still lost on the whole after credits ending. Does it mean atleast one booker in the universe had a normal life
It means that either HAPPY ENDING: since all Bookers refused even going to the baptism, the choice was never made, and Comstock could have never formed. Thus, Anna is born, his wife dies, he racks up debt, but he still has his daughter.
In the BAD ENDING: it's a "new" Booker having a tear bleed-over of memories, leading him to realize that Anna is still gone, and he's 'seeing' another universe's Booker come to the end of the road. Thus, the universe is still undergoing the loop.
Personally, I can't see Ken being that fatalistic, but I don't know if he ever stated which Bioshock ending was the 'canon' one that he made before he was asked to split the ending up into good and evil decisions.
also, the whole die in baptism seem unnecessary, why must booker die? he can always walk away before the baptism even happened. Killing him in one universe doesn't mean the other bookers in the other universe won't go through with it. They'll need to literally erase booker's existence at its root to clear all versions of himself.
But even if that Booker walks away, other Bookers may make a different choice based on small variables unique to that universe. The only way to ensure Comstock's non-existence is to kill all of the Bookers who even attended the baptism in the first place.
Basically, Ken's playing hard and loose with the infinite universe theory to state that while the universe moves in a relatively constant/fixed direction, there are some key variables which can change the way things proceed.
And while Liz probably could erase Booker entirely, that'd be the quintessential Grandfather paradox. It isn't conveyed well, but IMO, she is using her ability to see every single possibility to find a way that will only erase Comstock.