I am an adult (30+) and I don't think The Sims is childish. Humorous, yes. Campy and fantastical, sometimes.
Firstly, difficulty of the game the way I see it, depends on how much work I am willing to put it. There are cheats available ("shortcuts"?) and all kinds of challenges that the community has carefully crafted. Kudos to anybody who has completed 100 babies challenge! I don't think I could do it.
For me, my goal is generally "Super Sims" getting them to the top of their career (hopefully before their senior years) plus getting to max level of their aspiration, plus socializing with their friends (my sims are required to have 1 bff who is not related to them), pets and other family branch without hopefully too much cheating (no money cheat) or mods (no faster shower or faster homework mod for example) with autonomy on, aging on and default lifespan. I spent a lot of effort on a gardener sim once who work as a gardener, maxing out gardening, flower arranging, all the cooking and baking skills (all those veggies and fruits need to be consumed), fishing skills (fish make good fertilizer) and collect/splice/grow all plants, trees and flowers. Along the way, she fell in love, had a spontaneous wedding at a romance festival, raised children, 2 dogs (her and her husband walked them daily), saw her granddaughter born, before finally died from old age a day after I accomplished her (my?) goals. It was a lot of work and patience from my part (and even more from her) and I missed her dearly. Did she influenced her slice of world? I think she did, she and her husband never strayed from each other (and that matters to me), she raised children, her family always gathers at her place for holidays, ate countless pies and turkey. She comforted other sims, she tried to fix strain relationship.
In other word, what I am saying is: the maturity/complexity of your sim depends on your creativity and involvement. This is the idea behind sandbox games. Sandbox games should not spoon-feed you stories, they should instead provide a blank canvas for YOUR stories.
Secondly, I am not a romance/drama person (I do not enjoy reading or watching romance/drama heavy anything) so for me more romance, or more drama is not equal to "mature". To me the opposite is true: too much drama is immaturity. I love humour, I don't love drama.
Thirdly, Nuance and creative writing? That's very subjective, compared to what? Visual novels? Campaign/story driven games? Can you provide example of "nuanced" writing for a sandbox game? Most sandbox games don't have creative writings to be honest, it's more like: build this, plant that, make more things (money, widgets etc), make this number (population, happiness, etc) up and don't forget to pet the cows. Like I said: Sandbox games should not spoon-feed you stories, they should instead provide a blank canvas for YOUR stories.
Fourth, Animations? I can't think of any other games that has more varieties and complexities of animations other than the Sims franchise. Sports, city buildings, farming sims, strategy, hack&slash, fps, even the famed jrpg don't have that much animations (and jrpg is not sandbox).
Lastly, I think inventing Simlish is a genius stroke on behalf of Will Wright. Because again, the Simlish requires you, the player, to fill in the blanks with your own stories.
