Relationships that begin with infidelity rarely last. The statistics vary and I have found only book sources rather than formal surveys of statistical collections of data. A cursory Internet search brings up several sources misrepresented across multiple websites. How many cheaters marry the alienator and how many of those marriages eventually fail? That’s what you really want to know.
Dr. Jan Halper surveyed 4126 men who were successful career professionals. 82% (~3383) of the men she surveyed cheated on their spouses.¹ 3% (~124) of those surveyed eventually married the affair partner. ² This was a specific survey that focused on high-level career professionals–numbers may differ for men in different industries and at different professional levels. The 3% is out of the entire 4126; 18% (~743) of those men claimed uninterrupted fidelity, thus the 3% is not 3% of those who cheated; that figure is (~124)/(~3383) or 3.7%. Those who reference Halper’s study sometimes fail to point out that it focused on a certain population of men–high level career professionals–and I did not find a single instance where it was noted that the 3% was from the total survey number of 4126 which included men who claimed uninterrupted fidelity. Halper’s study is helpful in that the survey size was large, but she did not offer data about the length of the marriages that resulted from infidelity. Did most eventually end in divorce? We may assume that to be the case, but assumptions have no place in statistics.
Annette Lawson is misrepresented as stating that less than 10% of men leave their wives for the affair partner in her book Adultery: An Analysis of Love & Betrayal. Here is what Lawson said regarding those who participated in her survey:

While over 70 percent of the faithful remained married to their original spouse, this was true of just over half of the adulterous, and the more liaisons a person had, the more likely it was that they would not remain married to the first spouse. If they did divorce, only about 10 percent…married their lovers…Given the number of liaisons in total, these forty-seven serious affairs represent barely 2 percent of all liaisons. ³

Lawson did not state the 10% statistic was relating to how many cheating men leave their wives for the affair partner; rather she stated that 10% who do leave their wives marry their affair partner. In This Affair is Over!! Nanette Miner and Sandi Terri report that “Most affairs do not end blissfully, with the man and girlfriend together. Of our survey respondents, less than one percent of the men left their wives for their girlfriends–although nearly 53% said that they would. In some instances, the man did leave his wife, and yet still did not commit to his girlfriend.”⁴
Now most of you are already in the situation where your spouse has left and for many of you, your MLCer seemingly left for the alienator and they are now living together even while you remain legally married. From your place it doesn’t matter what percent leave their spouse, you are already in that number, so how many who marry the affair partner eventually divorce? Sorry, but I have thus far found only one reference repeated multiple time across the Internet.
References to Frank Pittman state that 75% of marriages that begin with infidelity eventually fail. This is referenced as though it comes from a formal survey. Here is what Dr. Pittman actually says:

There is something inherently doomed in those marriages that began as marriage-wrecking affairs. It is possible for them to work, but it is unlikely they will do so.
In my practice, while over half the people who get into romantic affairs end up divorced, only a fourth marry the affairee. Even then, three-fourths of those romantic marriages end up in divorce. There is a greater likelihood that the divorcing partner will be back with the original spouse in five years than that the romantic affair will be a stable marriage at that time.⁵

He does not state that his numbers are from a formal survey, but rather they are from his patient samples and those referencing Pittman ignore that he states that a fourth (25%) married the affair partner–much higher than the other surveys just mentioned. Pittman does not give the sample size–the number of patients in his practice. This was not a flaw on his part, he was not making claims that this statistic would hold true under rigorous testing, rather it is the fault of others who have taken this reference and implied that it came from a formal survey.
Most affairs do not result in marriage and of those that do most will end in divorce. That is true for your MLCer as much as it is for someone who is not in a midlife crisis. It is true even though your spouse hates you and is head-over-heels in-love with the alienator. Just because your spouse seems to mean it that she hates you and is in-love with someone else and will marry him it does not make it true; most say those things and yours is not the special exception that is more stubborn or more in-love.

Sources

1. Halper, Jan. Quiet Desperation the Truth About Successful Men. New York: Warner Books, 1988. page 205.
2. Ibid., page 22.
3. Lawson, Annette. Adultery: An Analysis of Love & Betrayal. New York: Basic Books, Inc., Publishers, 1988. page 287.
4. Miner, Nannette and Sandi Terri. This Affair is Over!! Bristol CT: BVC Publishing, 1996. page 21.
5. Pittman, Frank. Private Lies. page 246-247.