>How exactly would we lose the electric grid?

Coronal mass ejection

>Also, (nuclear aircraft carriers and submarines|jet aircraft engines) can be (very|relatively) quickly brought into duty producing electricity locally in every city, way before any pandemics and mass starvation.

That doesn't scale.

>Longer term, converting from the refrigerated fresh food system of today, to a system based on pickling/curing/canning/iceboxes/etc. is hardly black magic.

>In the short term hundreds of millions would starve or die from heat or cold. Cities only keep on hand weeks worth of food.

Nuke subs are my mental woobie against my top horror scenario, the low-altitude EMP detonation. Run silent, run deep.

However, this response does not scale well.

The grid would probably be there, non functional. It takes a lot of effort to maintain and you can't just exclude some capacity and hope the rest works.