In fact, the market price for AOE today is less than what the...

In fact, the market price for AOE today is less than what the Shell cash offer + Dart shares is worth, since the Dart shares are almost certainly worth more than 40c each. That means the market thinks this is the final offer, and is pricing in a risk that the takeover will not occur.

Another point to consider is that only Shell can give shareholders the Dart deal, since Shell have a complex option arrangement on the Arrow International non-Indian projects. Another predator would have to come up with extra cash just to match the Shell offer, and even more cash to trump it.