That's a toughie to get data on. A quick Google has separations in general (probably including some MLC) with a 15% reconciliation rate, and remarriage after divorce in general (also probably including some MLC) with a 6% rate. Not high numbers, but not non-existent.
If you look at the cumulative, very non-scientific sample here on HS, the odds aren't good. When I came here in 2012, there were far fewer posters, and many more reconciliations. But...the site had not been around for long, and most MLCs looked to last under 5 years (3 was really the average, from what I gathered, at that time). Some of those reconciliation stories from that time ended up breaking up again, so to me, it's all far more complicated than the tunnel story would initially have us believe.
Most reconciliations do seem to happen earlier on. The people who have long-term relationships with their former spouses (ie they weren't vanishers) seem to have formed new friendship-type relationships. Those are transformations of sorts, even if they're not the ones people were standing for. Those have never really been tracked here (maybe they should be? It's not discouraging to think that someday you could have a friendship again).
But, and this is not to be Debbie Downer, it really doesn't seem like it's a matter of people reconciling and then just never coming back to the forum (something else we've thrown around through the years). There could be those random cases, but in general, most old timers who were regular posters usually check back in at some point, and most of the news is usually more of the same, with their MLCers still lost out there in the mist.
It's so sad that this happened to any of us. But I want to be encouraging in a different regard. I used to spend every spare minute looking for returns, data, or any info that would keep me in there for my former spouse. And now, it never crosses my mind. Full acceptance, and it feels good. I have my life back. So even if they never get out of MLC, it's possible for *us* to get out of MLC.