Well, if you have no way to tell whether something is going to happen, or not, you don't prepare for it- because you can't justify spending the resources to prepare. Or rather, in a world of limited resources, you can't prepare for every single event that may or may not happen, no matter how important.
To put it plainly: you don't take your umbrella with you because you don't know whether it will rain or not. You take it because you think it might. Otherwise, everyone would be going around with umbrellas all the time, just because it's impossible to make a completely accurate prediction about the weather and you don't know for sure when it will start raining until the first drops fall.
In the same sense, if there's no way to tell when, or if, AGI will arrive, then it doesn't make any sense to start preparing for it right now. We might as well prepare for an alien invasion. Or for grey goo, or a vacuum metastability event (er, not that you can prepare for the latter...).
In fact, if AGI is going to happen and we can't predict it in time then there's no point in even trying to prepare for it. Either we decide that the risk is too great and stop all AI research right now, or accept the risk and go on as we are.