People are complex, relationships and feelings are complex. The person who was left will always hope for "karma" and fantasise about how much happy he/she is and how much unhappier the cheater will be in their new relationship. The upheaval of being cheated on, especially when there are children involved, is massive and becomes a central part of the "cheated on" partner's life which will always hurt.
But the reality is, some couples are not well suited to one another, and sometimes a new person appears who is suited, and meets at the right/wrong time when one of the partners is in a low emotional time, or very relaxed because they never thought they would fall in love with someone else... and falling in love completely changes perspectives, changes plans and what the person thought he/she would ever do.
I only cheated once when I was a teenager with a boyfriend of 5 months, never as an adult ion long relationships. But I have seen enough cases to know that is it never as simple as "the OM/OW will never be happy, there will be a vacancy, they will never feel good"... I am sure many people want to believe that, but it's bullshit. Partners who never cheat can also go on to have shit relationships. OWs and OMs can go on to have an amazing relationship. Karma, no matter how much we like to believe in it, does not exist. Plenty of Nazi criminals died calmly in their bed in their Caribbean house surrounded by family and loved ones after committing the worst atrocities imaginable. Sometimes bad things happen to bad people, sometimes bad things happen to good people, and the same with good things.
What I do know is, keeping grudges and being bitter about anything, cheating included, and keeping thinking about revenge and karma years later, hoping that the other person lives a miserable life, only hurts the person who feels like that. The cheater may well be living the best years of their life, or the worst. This should not make a difference in yourself striving to live the best life you can and putting hurtful feelings behind. Turning the cheater into a "monster" may provide short-term relief and feelings of fairness... but long term it is just not useful. A person who cheated on you may still be a fantastic partner for someone else, a great son, a fantastic friend... Just not a good partner for YOU in particular circumstances, in a particular point of time. Accept this, move on, forgive, be happy.