First of all because it makes the assumption that all of them are stupid somehow - or for the less adaptable ones that have problems with newer technology, it makes the assumption that the current status quo works. Do you think that granny from your example wouldn't click "Ignore" on a browser warning?
Second of all, if we really get down to an argument about elders, society and making the world a better place, the priority shouldn't be to keep the status quo because the elders wouldn't cope with change - because in that equation, today's children are more important, don't you think?
> Have fun explaining to her that she needs to exchange keys with enough trusted intermediaries to have a valid trust path to her bank
That's false - she only needs to exchange keys with the bank directly.