Not for nothing did Marvel Comics Group immodestly call themselves “the House of Ideas”. They ran a series of stories, over 150 in all, based on the concept of -what if something different happened at certain crucial stages of stories already published. The magazine, although uninspiringly entitled “What If…”, ran for a long time because the scripts were so good. The writers could run riot with the established Marvel universe and kill off major characters as they created alternative realities. They were fictionalising what was already fiction. Brilliant, if you ask me.
Coming back to reality we have all wished we could have made different decisions and turned that unrelenting ticking clock backwards just like we do in autumn time. At the moment it is not possible to revisit pivotal moments in our own little personal history. Maybe as progress progresses, in the future we might be able to go back to the past. Then, no one need suffer from l’esprit d’escalier; we could all be first floor repartee artistes.
On to the bigger picture there are some major what ifs that could have changed the world immeasurably. What if Kipling never wrote if? What if Apollo Creed had finished the bum and went home? What if at the end of Lonely Street there was no Heartbreak Hotel? What if William Webb Ellis could have applied himself and learned to kick a ball?
Boadicea's Chariot 
I am rereading ‘The Diary of a Nobody’ a brilliantly funny book. Mr Pooter is frequently regretting the fact that he only thought of a witty riposte when the moment for making it had passed. But I have never heard it called ‘l’esprit d’escalier’. I like it.
What if, ah, an interesting question, JW.
What if I hadn’t read your post? What if Bearsy and Boadicea hadn’t opened the Chariot.
Scary thought. 😉