I think one indicator of whether it will last or not is if the unfaithful spouse leaves their partner or is forced to leave when the affair is uncovered.
I'm not sure how reliable the statistics are, but I've read lots of times that a person genuinely in an unhappy marriage and looking for an escape will do so within the first four months.For every additional month that passes, it is less likely that that person wants to leave.
So when someone is discovered in an affair and is kicked out, and then by default ends up with OM/OW, it is less likely to work out because they were not explicitly chosen iyswim.
The trouble is that the OM/OW won't be told that will they? When their lover turns up on the doorstep or expresses relief that they've been told to leave, their new partner will believe it.
FWIW OP, my ex regrets it hugely and I think that's quite common. Real life is not like the excitement and forbidden fruit of an affair, where you show each other the best version of yourself for the short periods you're together. One day your new partner looks as dull as the old one, but with inherent trust issues, guilt, shame and all while handing over a chunk of your monthly salary and seeing your kids every other weekend.
The only woman I know who is in a proper relationship borne out of an affair is still jealous of his wife - the shared memories, long history, marriage, children and even the fact that his family still love and include his ex.
I would rather be you than her. Forget her, forget them, focus on moving on.