I’d be more suspicious of the router manufacturer or even a disgruntled worker there than anyone else to be honest. An actual hacker I think would be more motivated to turn them into a botnet or similar.
Can’t think of why someone would want to brick them without another motive but I could be wrong.
Call me skeptical I guess but if you wanted a phone that worked as long as lithium say 5000mah using capacitors I think it would be a dozen times larger at least.
Maybe I’ve got it wrong and supercapacitors hold more energy or something like that.