written letters are asynchronous but people expected timely (relative to snail mail) replies even back then.

I am pretty sure this is not true.

I recall my mother’s family conversing via mail in the early 80’s - and she would write one 10 page letter a month as a reply (max) - that would 3 or 4 mails a year with any particular sibling (and probably 1 phone call - but phone calls to alaska were expensive, and you wouldn’t say all you wanted to).