This is a very difficult question for me to ask... Because i generally disagree with the whole 'dumbing down' argument. Modern computer interfaces are absurdly 'dumbed down' from the original binary inputs, but does that make them worse? No, not by a long shot.
TES games have streamlined extensively. That is impossible to argue against. They've cut things. Again, you can't argue with the fact. But have they 'dumbed down"?
If we assume that the basic premise of TES games hasn't changed, then yes, they have. If we assume that the premise has evolved over the last (almost) 2 decades, well... no.
The second question, though (as i feel there are, in fact, 2 questions being asked here) is whether or not they have less depth... Again, yes and no. Does Skyrim come with less features and options than Daggerfall? Yes. Definately. However, when you take into account the CK, and what can be done with it, you have more options than you had before.
Really, i don't think i can say yes or no to either...