I've been thinking about the way that they had Will and Grace essentially apart from each other for 18+ years. I think it had to be that way.
The entire premise of the show was that Will and Grace were each other's everything - you'd want them together and married and happy if Will weren't gay. They were, for all intents and purposes, each other's spouse, and the reason why none of their relationships worked out. They really didn't write many of their breakups to be specifically because of each other - but each filled the void the ohter had for a long term romantic relationship.
So in order for their marriages to work to Vince and Leo, they had to be out of each other's lives. When Will had a really big moment, he needed VInce to be the one he'd share it with - not Grace. In most best friend situations, it's easy to have both. But the two of them were so connected to each other, that it wasn't the same for them.
In some strange way, I think that they're being apart for 18 or so years had to happen to make their marriages work - and they both deserved happy marriages.