As soon as he finds himself in a position in which he decides that he has the moral right to look beyond MsHD for sexual partners he will immediately lose his attraction for a woman who is so cold and callous in her sexuality.
I see your point. It's certainly possible.
Did I miss something... where did we get the idea that you and HD are similar?
Your STBX will probably find some 25-year old Goth with black fingernails, coming off a meth addiction, death-white skin, no job. He'll meet her when she is standing on a bridge at 3 in the morning getting ready to throw herself off, and he'll talk her down, take her for scrambled eggs and coffee. Then he'll find that she's escaped from the rehab facility where her rich parents keep putting her... iow she's Winona Rider in "Girl Interrupted" What I'm getting at is he'll find someone sadder than he is who makes him look/feel healthy by comparison. After all, the 4 in decline becomes a 2, the caretaker.
Or maybe he'll find an ex-matron from a prison... someone who's as strong-willed as you (with the same body) but with none of your jolliness or kindness. She's 15 years OLDER than he, and she'll grimly set about the mission of turning him into a goose-stepping productive member of society, with a 9-5 job, 5-mile runs before dawn, and a diet of rare meat and raw vegetables.
BOTH of these women will make him long for you and wish he could turn the clock back and do everything differently... but that's the natural, comfortable zone for the 4 anyway, so actually it's a win-win sitch all around.