I think HVDC is a more important component in smoothing out demand/supply than you give it credit for, especially if you add wind into the mix.

In terms of security - one of the reasons nuclear power stations are so expensive is they have to survive a targeted plane crash etc - they are expensive high profile targets.

In the end the renewables model is a much more distributed model of generation, storage and consumption ( rather than a few massive power stations ) - so with a proper grid you could argue you would have fewer single points of failure, and increased resilence.