'why is the phrase 'old folk' offensive?"
But it isn't. That wasn't the issue.
One issue was - I thought - very simple: an entire cohort of people were defined by age but then characteristics were applied to that group as an entity. It was as though
all people over 65 behaved in the same way. That, to use Yorkie's eloquent turn of phrase, is "one large mouthful of garbage".
If you're going to assume all over 65s behave in the same way, then it not only displays an astonishing lack of awareness about an entire group of people but could be seen by members of that very group as highly offensive.
I know it can, because - like you - I've had emails and chats with members who feel exactly the same.