> I suppose only aimless sandbox games escape this.
Flow theory suggests that players will self-direct to find challenges of their own making. A sandbox game needs to provide enough opportunities for that and make it suitably rewarding.
> The gameplay loop of Counter Strike is nothing at all like the one in Obra Dinn or Return to Monkey Island.
No game has one single gameplay loop, they're all meshes of smaller loops, but as you go deeper you always(?) end up seeing the same structure, almost fractally. The reason that I believe this model applies to both Counterstrike and for Monkey Island is that you can understand the difficulty progression for both games in the same way.
If the test of skill for aiming and shooting is primarily being able to move the left stick precisely and pull the right trigger at the correct moment, then I know that the challenge will be harder if there is more precision required (make the range of valid x/y position values returned by the stick smaller, e.g. by reducing the size of the enemy on screen - distance or physical size); or by reducing the timing window in a similar way.
In a linear shooter game you will use this (applied to every mechanic) to steadily increase difficulty throughout the game. In Counterstrike you can use it to balance and measure relative weapon effectiveness and also analyse why some positions are easier/harder than others.
In a puzzle game like Monkey Island (which predates all this theory) you provide the player an objective and put a number of complications in the way[1]. You can make a puzzle harder by adjusting the number of complications in the way. You can also adjust the difficulty of each complication. If one puzzle type requires observation skills you know you can make such a puzzle harder by making the object harder to see on screen: size, shape colour, animation, etc. If there is a memory test component, you make the puzzle harder by increasing the number of things you need to keep in working memory at one time.
You can then reuse those puzzle types again and again, controlling the difficulty of the challenges over time by increasing the difficulty of individual skill parameters and combining them with new ones. The same thing you would do in a linear shooter. The Witness is a perfect example of this.
This is not my own personal theory by the way. It's also not the only way of looking at game design, but it's a practcal method I've seen all over the AAA space, at least. It's not strictly formalised across the industry but Ubisoft internally called it Rational Game Design[2].
[1] https://www.youtube.com/watch?v=zsjC6fa_YBg
[2] https://www.youtube.com/watch?v=xqHAjwrNp70