Shadow of Mordor is a clear cut example of a game that had to sacrifice certain gameplay mechanics (the nemesis system) to be able to run on last gen hardware. Not everything is 'scalable'.
I don't know that Shadow of Mordor is a "clear cut" example, as I'm not sure what
had to be sacrificed in order to get it on last-gen hardware?
The Nemesis System is cool and complex as an amazing spreadsheet tapestry of possibilities and interactions, but I feel much of it could have been done (or certainly done better) on last-gen hardware (though
Monolith says otherwise,..) they just didn't budget and scope it out for that. They farmed past-gen out to an external developer (a developer that regularly churned out remake-ports instead of carefully converted port-downs, including Mercenaries 2 PS2 and Dante's Inferno PSP,) they had a game built on an in-house engine (Monolith has licensed out its LithTech engine, and of course LithTech does run on the last-gen hardware of the time, but I believe SoM was ported using Behaviour's own engine at the time?), and they just didn't prioritize the past-gen versions as important SKUs compared to the one Monolith worked on itself.
Things are a little different now. A game being made in Unreal Engine 4 or current Unity can be used to build multiple versions of the same product, and even tested to see how a build and test scenarios would generate on different hardware. (I would assume Slipspace Engine had both Xbox One and Xbox Series X testing systems.) And if you have one of the popular development suites, you can work with people across the globe and share assets and code without forking the project to death every time somebody in a different timezone makes a change. A developer couldn't faithfully port Shadow of Mordor if it truly was impossible, but you wouldn't just be handing off your assets and design docs to a studio off in another country and saying, "Good luck!"
You are right, at some point you scale beyond the range of the console gap, and then not everything is 'scalable'. But most PC gamers (I'm a console guy, so I'm just going by min/max specs) would say there's surprisingly a lot that can be done within that range before the gap is impossible to jump. And that's just for pure runable code on flexible machines; get an expert porting studio to carefully target a set console spec and it's possible to work wonders... just not miracles.