I wanted to write my opinion before reading anything here so my ideas wouldn't be affected by other comments yet.
I'm glad they didn't kill Alex. First, because killing off characters is way overdone on this show. Second, because I think 99 percent of viewers expected that to happen. Way too predictable.
IMO the writers did pretty well considering what the cards they were handed. I would never have believed Alex would go back to Izzy, but they made it believable with the flashbacks, new scenes, and his explanation about why he contacted her.
It's almost like the writers had foreshadowed this a long time ago, with Jo's concerns about the embryos and how he didn't seem to want kids with her. Even though Alex supposedly wanted nothing to do with Izzy every again, he built up an elaborate happy fantasy life for her.
As he said in Jo's letter, this is the worst thing he has ever done. I even repeated that back to the TV. "Yes, Alex, this IS the worst thing you have ever done."
It creates the possibility of a more interesting story for Jo as how she will deal with this. The show has already done too much 'grieving partner' because of death.