The problem with that logic is that it was all perfectly preventable or at least could have been mitigated if certain things had not happened during Elly's second pregnancy and Lizzie's first year of life. The first thing, of course, would have been for Elly to not continually rant about how great it was to become a mother again and how lovely it was to have a new child within earshot of the younger without taking the time to tell him that there was still a place for him in the family. The second thing would be not act react is if she'd been asked to disembowel herself with a salad fork when he comes looking for reassurance and the third thing would be to try to remind him that other people's feelings are real.
This, of course, would have required Elly to behave as if she believed that what happened during childhood had lasting effects; while she herself reserves the right to resent Phil forever for all the crap he pulled while ignoring the fact that she gave as good as she got, she seems to not have realized that the rather vicious infighting, the name calling, the teasing, the bullying and the nastiness had any other effect than making her look bad in front of the neighbors. The extreme tunnel vision that seems to be her signature mental defect, of course, seems to require that she can only view the concerns of people in the exact same predicament as she's in as being in any way valid.
What would also have to happen is for Mike to listen to a voice that's been in his head all along; the same voice that told him that he was being a jerk to Martha and Rhetta because his stupid pride was hurt is also trying to tell him not to behave like a swine around Liz just because he feels left out; just as he never asked to be born to a father who selfishly assumed that he was owed every penny he spent on his children because he did what he was supposed to, Liz never asked to be born the younger sister to an insecure twit who lashes out because his feelings were hurt and because he was asked to be primary caregiver by someone who doesn't know what she's doing.
The sea change would, I assume, happen when he and Deanna started noticing that Meredith's grades were starting to suffer and, well, they'd gotten a lot of calls from teachers asking if they would kindly come down to the school to discuss a problem that their eldest won't talk about. Said problem, of course, would be that Merrie had been the target of the same sort of bullying and name-calling that Brad subjected Mike to back in the day. Under Lynn's hand, the whole thing, of course, would turn into an opportunity for leaden moralizing about how losers need to win and similar mush.
In the hands of a competent author, however, it would have finally gotten Mike to remember that he'd been an ass to Liz all his life because of things beyond her control; he could no more hide behind the petulant and self-serving whine that she'd deserved to be punished for being cuter than he could fly without the aid of wings and an engine. I, of course, don't expect him and Liz to suddenly (or, for that matter, ever) be best friends but what I do expect is for him to finally see the past clearly enough to know that he was a clod who could have and should have done a lot better.
We might even end up with a parallel to Funky Winkerbean; as you may or may not know, one of the former mean girls went around trying to apologize to a woman she'd picked on back in the day. What happened was the other character, while acknowledging the sincerity of the emotion, wondered what the point was; it's not as if the pain could be redacted or if it mattered any more. Batiuk didn't exactly, of course, come out and say "Repent and sin no more" but that seemed to be his intention; we could well see Liz dealing with the baffling need Mike has to atone for things she no longer cared about. The person who taught her the lesson that she needs, of course, is the subject of my next article: April.