There is no way that Google would suddenly start to mark as spam continuations of threads that were perfectly fine between senders that have been in contact for years unless their spam filters were either seriously broken or they tweak them to cause more people to switch to gmail.
When you refer to specific threads going back and forth this may be. But here's my counter example.
I sent emails to my girlfriend's gmail address from my own domain hosted on a mail server run by a reputable company for years with no problem, whatsoever.
Then, on vacation in Thailand, my rather elaborate trip reports for which there's really no plausible explanation that they could be spam, suddenly bounced.
Sent from the same domain from which I sent mail for years and Google suddenly branded me a spammer and refused to deliver those mails even to her spam folder.
The auto-reply from Google is about as useless as you would expect. There are a few links, which are neither informative nor do they help to resolve the issue.
I worked around the problem by sending mails from my own Gmail account, but it left a really rotten feeling around the lines:
NICE COMMUNICATION YOU HAVE GOING THERE, WOULD BE A SHAME IF SOMETHING HAPPENS TO IT!
So I guess your assertion is wrong. Our exchange went on for a minimum of four years. Always from the same account and suddenly I was deemed an evil spammer.
It really sucks.
or they tweak them to cause more people to switch to gmail.
That, actually, is my suspicion. It's easy to brand that as "protecting their user base", but in reality make it so cumbersome that you're not able to send mail to a significant percentage of users over which Google controls their email unless you switch to Google for email.
Exactly. Look at it this way: an account with good reputation for years, suddenly showing up from Thailand instead, is exactly what a spammer hijacking the account would look like.
Regarding a specific thread going back and forth (span of a month or so, not years):
Google marked as spam my 7th or 8th reply in a chain of emails with a prospective tenant (presumably) because my email contained instructions for sending me money. I assumed that they weren't interested anymore and had ghosted, but luckily we crossed paths at a grocery store or some such.
My emails continued to be marked as spam after they added me as a contact, individually fished each one from their spam folder, and continued to write replies. Weeks later (with no apparent cause) I started arriving in their inbox again.
