It's an interesting take to be sure. I suspect that the lack of flexibility is going to be the real killer.

You'd probably have to build offshore platforms on either side to bring the cables up and terminate them and now that's a nightmare, saltwater/salty air and electronics don't mix well.

Or you're going to have to trench very deeply for the first few miles.

Either way you're stuck with something that really doesn't want to be bent.

I think the "glass is great insulation" is a good insight and perhaps a composite glass fiber/polymer sheath would really increase the V/m without the brittleness.