I don't think the problem is rooted in cognitive overload. There aren't too many things competing for my attention in this game and I sure wouldn't want devs to think they need to remove even more stuff from the game. Sims 1 was fun when I was a kid, but I can handle more than that. I loved sims 3 and would still play it a lot more if it didn't crash or bug in unacceptable ways so often. It had just as many systems to pay attention to if you had all the expansions, and that never really bothered me. Impermanence... yes.
I think the problem lies mostly in lack of consequences, as others have said as well as you, in a way, and repetition. Those are tied together. Lack of consequences comes from some systems not interacting enough or some systems (emotions) taking over everything else. Often emotions will override traits in deciding how your sim will act and what they want to do, taking away the weight of some of our choices. They override places and events, making sims go jogging or do push-ups when they are supposed to meet their family or visiting a museum, just because they are energized. Emotions have also sort of replaced memories, so now, if two sims fight, they will be generally angry for a few hours, but they will basically be angry at everyone, and once they have gotten rid of the moodlet or simply overriden it with happy moodlets, it is all forgotten and they can happily joke around with the person they just had an argument with...
This contributes to repetition : sims often feel very similar despite you making different choices, since most of the time given events or activity will give them all the same emotions and whims, making them act similar in the end despite different traits and experiences and despite your wish to give them varied interests. Repetition also comes from checklists. In events, life-time wishes and careers, your sims have to accomplish the same goals no matter their traits or interests, which means once you've tried doing the career once, it will mostly be the same the next time. You get less choice than in previous iterations. Dates have to go through certain steps, and other choices you make barely matter (you kissed before it was on the list? that doesn't change how your date went). All your entertainer sims will have to learn an instrument and some humor, despite one being mostly a music lover and a loner, and the other being a clumsy oddball, for example. In Sims 3 (just for an example of what I mean), my sims could climb the ranks because they were skilled, but some less skilled sims could achieve it simply by making enough friends with coworkers or doing the tasks their boss gave them. There were many different ways in which you could achieve something. Similarly, with long-term wishes, in Sims 4, all sims will have to go through all the same steps in the same order, for the most part.
I don't look at moddlets much because I've seen most of them very often, so I just glance at them once in a while and read the new ones that are triggered by new dlc content. That's not a big problem, really, but the thing that does show a problem is the other reason why I don't look at them: in the end, only the strongest emotion matters. That sad moodlet has no effect as long as it is burried under happy moodlets, so I dont really need to pay much attention to it unless I haven't done anything to make my sim happy in quite a while. I didn't go looking for whims much because there are always some whims that come back too often (buying toys because they have a pet, watching the City Living tv channels, meeting new people, despite your sim being a loner or having a ridiculous amount of acquaintances). Meanwhile, sims get fairly few whims truly pertaining to their traits or long-term wishes. That makes it so that, again, sims don't feel distinct enough from one another.
