While I agree with your conclusion, I disagree with the method used to reach it.
My main issue with arguments along the lines of "Heavy armor should make you run slower/not be able to cast magic" are that they are nonsensical. They take issues that are correlated, but not necessarily caused by one another.
For instance, it is the armor's weight which makes your character run slower. So a character wearing 100 pounds of steel armor (and nothing else) would run the same speed as someone wearing 10 pounds of fur armor and carrying 90 pounds of loot. (Okay, generally speaking, light armor IS more flexible, so in theory you would still run a little bit faster....BUT take into account that even stiff steel, molded to your body, is easier to move with than a random bag of weapons, gold, alchemy equipment etc)
On the other hand, stuff like magic. It makes sense that most wizards would not wear heavy armor. Is this because armor has a random, anti-magic effect? NO! It is because a mages main attributes are Intelligence and Willpower, and the Attributes needed to wear Heavy armor is Endurance (or Strength?) Either way, they are unrelated attributes and so the mage is diluting his abilities as a magic user in order to gain that advantage. Some may choose to do this, and that's where you get cool hybrid classes like battle mage and it's not all just Fighter/Mage/Thief.