This is probably a minority opinion on GAF, but a pretty even one if you used to buy these games every year... the lack of a modern story post 3 really killed a lot of drive from people to have a reason to buy these games every year.
I know a lot of people don't like the modern day, find it annoying, a immersion breaker, etc. But for others including my self formerly, it was a hook for buying these games every year. Like a TV show, where each game was technically a season. Not to mention the expanded cannon stuff like the modern day stuff from the comics was actually really interesting, and outclassed anything shown during the actual games.
So after 3, this complete lack of anything actually happening in the modern day has killed a lot of interest from people who were buying these games yearly. Black Flag imo was a tounge and cheek break, and while I thought it was nice at the time, there was no way I would stand with it for more than a year, and Unity's modern day was literally "oh, Arno did the thing... guess this was a waste of time" and you are literally just some dickhead who bought a video game.
And the syndicate creative director is saying "oh, there is significant modern day" "you'll love it" "I promise"... but they have showed fucking nothing, and people have very little confidence and worry it just going to be a repeat of unity.
The only thing we know about Syndicate is
And honestly, this constant change of creative directors is fucking harmful. One year they go "oh, we want to be authentic to this culture and time period as possible" to "fuck it, everyone has british accents because its "cinematic""
All these writers, this next 10 years of AC planned has no cohesive vision. Its practically all fan fiction.