Atmos Duality said:
But instead of improving the system with known depth, Enemy Unknown threw out everything and forced all mission mechanics to work on a 2-action system. Which mathematically offers less action fidelity than the Time Unit system.
That, objectively, is the loss of depth in design space. The player now has less control over their situation, and the game is more limited in what it can provide in terms of tactical challenge. The potential skill ceiling was lowered, ergo "dumbed down".
This is going to be the only part I respond to because this is the root of the issue.
Complexity does not result in depth unless that complexity is well utilized. In fact, complexity that is not well utilized results in a reduction of depth. And XCOM is highly complex but that complexity is not effectively utilized. The key, as I have been saying all along, is meaningful decisions.
Lets take a specific example, the time unit system. Yes, there are mathematically dozens more decisions or outcomes to the time unit system compared to the two action system. But generally speaking you accomplish the same things in both. In fact, the amount of meaningful decisions are reduced because there are so many options.
Lets look at one specific case where this happens. In the two action system, attacking an enemy out of sight requires you to break cover. You must make the decision of leaving your unit in a safe position or moving them to a more vulnerable position but allowing them to fire. In addition, choosing where to fire is important. The optimal firing position is rarely the safest one, and so we must make another meaningful decision: Safety vs Firepower.
In the time unit system all of this goes out the window. It is easy to take a step out of perfect cover, fire, and move back into perfect cover. You don't need to make a tactical decision of safety vs taking a shot. In addition, deciding where to fire from is a far less important decision. You can move after you shoot so where you are when you fire is much less important. The overall process has far more options available but is also less meaningful.
In this case the most important decisions in a strategy title, deciding when and how to attack, are less deep in the time unit system because there are so many options available. A tighter more limiting system forces more interesting and meaningful decisions.
And there are dozens of cases like this. Deciding how to outfit your troops, for example, requires far more meaningful decisions in Enemy Unknown because there are strict limitations.
Greater fidelity mathematically results in more options but having more options can itself be a barrier to depth.