I hate to defend Ubisoft, but this isn't fair at all.
That "splash of paint" is the world design of entirely unique locations, a full story, a cast of characters, and new arsenals of weapons.
As an amateur dev I have a bit of insight into this. I can, and have, made an entire FPS system in less than a day. A play that can move, weapons to shoot, and enemies that can target, follow and shoot at the player with the same weapon system. That part is not where the work is.
It took two weeks to build on that foundation to barely make one small level. And I didn't even manage to fit in any story.
The point is, those mechanics that to you are "the game" take infinitely less time to make than everything "the game" takes place in.
Anyway, you might be right, but you also might be wrong, it entirely depends on how they have implemented it.
They are from a gameplay perspective the same game. That ubisoft possibly does the sadly very common "to win the marathon you have to sprint faster", which makes it take a lot of work to spit out something very similar would not be surprising, but also does not change the fact that they are imo too similar.
I mean I'm not entirely in disagreement with your original statement. I don't imagine they change much about their gunplay at all, apart from adding some new mechanics each game, and they likely crank the core out quickly by either reverse engineering, refactoring or just copying the whole thing in.
My main point is just that the "coat of paint" which they do indeed throw over reused mechanics probably takes a whole lot of work. It's perhaps lazy thinking, but not lazy design exactly.
I'd also say, Immortals Fenix Rising was excellent, complete and bug free as far as I remember. It's too bad they dropped it right next to AC Valhalla and nobody played it.