As far as I know, MySims and The Sims have a world of difference. So it's sort of wrong to compare the two, when it comes to audiences. It seems like the stuff you get in MySims is more childish then the stuff you get in The Sims. Example? MySims: Normal Household items, Chairs, Tables, Appliances. The Sims: Heart shaped bed that has a "Vibrate" option, Hot tub for multiple people, Bars, Pools, and that Mechanical Bull. See what I mean?
MySims is directed at a younger audience then The Sims, but that doesn't mean you and I can't enjoy it either.
But as far as "dumbed down" goes, I think you made a mistake. You're assuming MySims and The Sims started out alike, but then MySims got more chilidish. Which is, in a way, true, but early screenshots/videos never really depicted this. So in actuality it was "kiddy" to begin with, but you nor I never noticed it.
So you assumed it was like The Sims. But with new photo/video evidence popping up, you realised it was much different. So you called it "dumbing down" when all it really did was stay the same.