My ex and father of one of children is still with OW nearly 20 years later.
They more children. I'm married and have more children.
I will say their relationship has been much more volatile than mine. Lots of break ups. Episodes of cheating.
Their blended family unit has had lots more issues than ours.
My marriage is much calmer. We 100% are with partners we are better suited to.
I am genuinely happier than I have even been with my life.
Personally I don't think I have to forgive my ex or his partner. They both treated me like absolute shit and made my life a living hell during their affair and afterwards.
I aimed for acceptance. Accepting the circumstances and letting it go and focusing on my child. I stopped focusing on them, their life and focused on mine.
I rarely think of either of them in my day to day life now.
Do relationships that start as affairs last? Like any relationship some do and some don't. If they last or not doesn't remove the damage they caused.
You are 100% right not to think about if it will last or not. If they broke up tomorrow it won't undo the hurt and damage that has been caused. He will have still had an affair, your marriage is still done and your children now will be co parented in seperate homes.
You are also right you owe your children the best co parenting relationship you can, but you owe him absolutely nothing. Especially not in relation to his future happiness.
You owe yourself the mental freedom and emotional ability to move on for yourself.