I can see how Woody and Soon-Yi getting together in the first place may have seemed like a bad idea, but this isn't a hypothetical situation where we can argue about what the outcome would be. They've been apparently happily married for the last twenty years, far longer than Mia and Woody were together and about twice as long as both of Mia Farrow's marriages combined. I'd say it was a good move on both their parts.
The fact that Soon-Yi and Woody have been married so long raises further doubts about Mia and Woody's relationship. The reason Eric Lax gave for them never marrying and never living together was that both had had failed marriages and Woody felt he was too neurotic or eccentric to live with anyone else. This obviously wasn't the case. He just didn't want to live with Mia. And if Mia was sleeping with Frank Sinatra the whole time, as she now claims, she wasn't terribly committed, either.
Back when this happened twenty-one years ago, Mia Farrow didn't attack Woody. She physically attacked Soon-Yi. Mia never acted like Soon-Yi was a victim---she acted like Soon-Yi had stolen her boyfriend.
When Mia adopted Tam a short time later, she cut out a picture of Tam and pasted it over Soon-Yi's face in a family photo (this was before Photoshop). Mia thought better of it and removed it before anyone said anything. It's now been reported that Fletcher has Photoshopped Woody Allen out of all the family photos----I wonder if he removed Soon-Yi as well.