JS777: well of course modern equipment has the full range. But Acars is lost when out of VHF coverage, roughly 100NM off a coast. Selcal is via HF. The only thing that's left is Satcom over the sea. Acars and Selcal can be coupled to Satcom. But if this last system is defect, you loose that too.
Remember Swissair SR111 Halifax, where they lost all electricals after an onboard fire. One could speculate that you still can fly 100's of miles without any communication link, and that makes tracking much more difficult.
I don't say you are not right, I just want to put everything into perspective.
Dani