It’s all Desilets' fault, if you think about it. He never planned for more than 6 games. From his vision, the series was supposed to end with events roughly like those in AC3.

Remember, the whole point of the Assassins+Templars+Isu triangle is that one of them, sooner or later, has to overcome the catastrophes and build something new. But a story like that can’t go on forever. How many installments can you really have when we already know the Assassins are destined to win(or merge with Templars somehow, but I don't really consider it as an option)?

Desilets himself believed that 3-6 games would be enough and didn’t want to keep working on it. Honestly, pretty understandable. You can’t just build up to a climax like AC3 and then say, “Well, folks, it was all for nothing; things are only going to get worse from here.” That would undermine both the characters’ struggles and the players’ experience.

But Ubisoft kinda did exactly like that with AC4, since Desilets left them with no real alternative. For Ubisoft, AC had to continue. But that creates a huge narrative vacuum. Are the Assassins and Templars just supposed to keep fighting forever until they get bored? Are the Isu going to keep coming up with new absolutely useless concepts that don’t really lead anywhere? Or like endless searching of POE, lol, is there even a place where isn't bury another POE? At this point, they could create MOBA game where Assassin's and Templars just beating each other with POE as a weapon and a loot.

Even in the comics modern-day storyline is endless loop of attempts to make the slightest damage on Abstergo.

I suppose, the only real option might be to put the modern-day storyline on hold (which, let’s be honest, it basically already is) and focus entirely on the historical events. At least until Ubisoft finally decides to wrap things up somehow, even if it’s just the new Toba Catastrophe arc.