I think I understand what you're trying to say and for Unity I agree with you. Also agree with you that Syndicate was kinda downgrade all over. But I just don't agree with you that Ubisoft did nothing to the AC engine Anvil since then and stayed the same.
I believe there are certain things caused you this thought ("
they downgraded the AC games so much, the engine never truly recovered from it.").
First is Ubisoft really did opened flood gates with the Unity and at that time's technology and PS4/XboxOne/PC couldn't handle that. Remember the negative backleash they recieved? With all those bugs and problems? They opened flood gates too much and rest is shitstorm. They even moved from Unity prematurely because of bugs. But they made the generational graphics jump. Hell they even outrun themselves again with the Dead Kings dlc.
Second is the decision to shift AC games to open world instead of semi-open world. When they leaped technologically with semi open Unity and caused all those problems, they wanted to play safe with their new open world AC game Origins. They didn't get too invested in graphics wise. They focused their attention on open world and gameplay. I think their choice borne fruit and Origins recieved good feedbacks both graphics wise and gameplay. But like you said it wasn't jaw dropping like Unity. Nevertheless I cannot say it was a downgrade after Syndicate. I think with Origins Ubisoft entered diminishing return area with Anvil.
Third is Shadows may seem the continuation of Valhalla/Odyssey/Origins on the surface but it seems there are lots of improvement under the hood: RTGI, upgraded LOD, seemingly highly reduced pop-ins etc. etc. That is why I'm trying to say I can really see improvements since Origins graphically. Since we are in the domain of diminishing returns, maybe it doesn't seen as much but as a mainstream game and even a Ubi game I'm impressed by it. Didn't expect them to improve an AC game this much. For me this is kinda Unity leap, unexpected from Ubi at all.
Maaaan I'm not a graphics person at all and writing here all this wall of texts

I know that most people hate Ubi and they are rightly so. I hate them as well on some occasions. But I'm also praising them for their effort with Shadows. I can both hate and praise them nothing wrong with it
PS: Again, I won't accept Unity is better than Shadows comparison. In totality of graphics, Shadows put other AC games behind itself. Ubi may found the sweet spot for their future AC games. The next thing they should do is improve LOD again, getting zero pop ins/ups and maybe add path tracing.