Its far too early to make a judgement on the overall quantity of heavy armor in the game based on a 20 minute gameplay demo. The elderscrolls have signature armor types, the 6 you have mentioned...but they also have armor types native to the land. And we don't know for certain how much culture will effect the variety of armor.
Silver was only weaponry in both Morrowind and Oblivion. Morrowind had a Silver helmet...but no other pieces. I think its kind of cool when partial pieces of armor are added to a game as it forces you to mix and match but it'd be cool to see a full set of silver armor.