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Okay, here's the spiel - Immersive Sim designates a particular school of design in Action games that pursue a comprehensive illusion of embodiment in the fiction at the mechanical level. Emergent gameplay isn't the goal in and of itself, but rather a critical requirement stemming from that illusory embodiment. As a concrete example, stacking boxes isn't desirable simply because it opens up alternative traversal options, but because someone facing a real world problem would naturally expect to be able to move objects and climb onto them to reach that damned spider up on the wall.
Nothing is absolute, but intent is key and that core vision for embodiment is what drives many other design decisions in an Immersive Sim. Emergent gameplay, first-person perspective, time-locked spaces, diegetic UI elements etc., they're all obvious and collectively harmonious design requirements stemming from that vision.
With Skyrim, it's obvious Bethesda prioritised other aspects. There's a lot of common ground, particularly where emergent gameplay is concerned, but TES progressed in the direction of creating a massive fiction, rather than a highly intricate one like Thief or Deus Ex, and this propagates across many facets of the games, from storming Windhelm with a legion of twenty to the collision model not letting you crouch your way under a ladder. And this isn't a diss, mind you, development resources are finite and not everything needs to be an Immersive Sim. I like Skyrim despite my various gripes.
As for Weird West - it's not so much that an Immersive Sim needs to have first-person perspective because someone wrote it down on a sacred tablet, but rather that if you've decided to make an isometric game, you're already labouring from a completely different perspective with completely different values - it could be a great game, but it won't be an Immersive Sim. We can debate the finer points with Skyrim, but not with this one.
I like Todd, he's a sensible chap. Bastardised or not, I wish more developers specified Immersive Sims being a big influence on them. Particularly some fellers up in Warsaw.