First, God forbid, I hope you're right and you or your Tracy doesn't get sick and die. It could happen though. But if either of you do, wouldn't you want to spend every moment you can together? And wouldn't you be happy for those moments? And you know what? You're right. You probably wouldn't end up back with your ex from ten years ago. But you can't say for certain. Losing your Tracy could change you. Time could change your ex. You and your ex could be two very different people and just be in the right place for each other.
But, don't you think Ted of April 2014... The Ted who's very happy and about to go back to Farhampton with his Tracy... Don't you think he would say the same thing you did about Robin? I admit, they did not do a good job explaining that on Monday night, but I'm pretty sure that's the intent.
I don't deny that it's a bit of a stretch Ted and Robin ending up together. The truth is, that aspect of the show was always hard to swallow for me -- exes hanging out. I barely even speak to my exes. But, the reality of the story is: people die. Sometimes unexpectedly. And Ted and Robin had reconnected over the years. Perhaps because of a Tracy before the wedding. Robin suggests as much in the last scene as MacLarens. Ten years later, that friendship, which was once a romance in this case, could turn into something more. Friends become lovers. It does happen. That's real as well.