Oh, I don't know - I think you might be interpreting that a bit narrowly. If you think 'what you might have been' means circumstantially (brilliant doctor, highly-respected scholar, contented husband), then of course it's facile, and wrong. Lydgate isn't (wasn't? It's difficult to know how to do tenses in this case!) going to be the famous medical researcher he might have been, but didn't he have a choice about how to carry on the rest of his life in the light of the previous crap decision? When he finally gets the emotional wisdom he lacked before, he chooses to be the best husband he can be, despite his knowledge of how rubbish Rosamund is, and how much happier he'd have been with Dorothea. And Casaubon definitely had a choice - he blew it, but he could have moved past his past limitations when he was given a second chance in Dorothea. If he were a character 100% fixed on an inevitable path, I don't think the narrative switch to his perspective would be at all as interesting.

Lydgate's understanding of his commitment to Rosamund's being absolute might not fit with our understanding today, but one doesn't have to apply the parallels narrowly in order to see that there's always an opportunity now to choose, even within the constraints of one's past choices and their responsibilities. Oh - look at Mrs Bulstrode? The possibility of ever again being the respected figure in her society is taken away from her, but she absolutely becomes much, much more in that painful decision to be loyal to her husband, taken at a time in her life when she might well have decided she couldn't give up everything she'd always thought important. She's a nice counterpoint for Casaubon, showing that it wasn't too late for him to etc.