Mine is that I can't afford to hire a team of engineers for three years...
Ultimately it still comes down to the question of whether or not you've got the chutzpah to free-fall for that long. I'm not sure I would.
I know right? I would've said "yeah maybe I'll do it given the chance", but a few weeks ago, I got the chance to jump from a cliff in a river -- very safe, and everyone was doing it, but I still froze for a second. Then someone behind me went "just look ahead, don't look down!=" which is what I did! :)
I'm not sure you should, either.
Excuse for what? There are plenty of ways to show you "still got it" not involving plummeting from the stratosphere.
Maybe the fact that (I'm assuming) you don't work at Google?