You are absolutely correct. The Smithing Perk Tree is designed perfectly, it only appears "broken" or "misleading" if you don't actually look at it up close. It makes perfect sense that you shouldn't go down from a level 100 perk to a level 90 perk of an entirely different class.
It makes sense that you'd need knowledge of the other heavy materials to be able to gain the knowledge of how Daedric material works. It wouldn't make any sense to go backwards from the light armor side of the skill tree, because your character effectively knows nothing about heavy armor and weapons.
It doesn't make sense that weapons, being not categorized in any way whatsoever, have to be locked with armor
Heavy Armor and Light Armor both already have their cons and pros, there is no need to cap light armor users at Glass, especially because between Glass and Daedric War Axe, there is no difference except for base damage and looks
If you think "Light Armor users already benefit from lighter weight" then I'm sorry to say that Heavy Armor users have a skill that eliminates the problem altogether, so the point is kinda moot at later levels, and at lower levels the difference of damage received is negligible between low armor points and high armor points
Smithing is flawed in that it does not provide Dragon level weapons, and thus Light Armor users are in disadvantage. Personally, I think either Dragon and Daedric should be switched, or Dragon level weapons should be introduced. Both parties WILL be happy with that
As about how come people can think of going around from Dragon to Daedric, I'd say that it has a lot to do with weapons not categorized in anything, and as such people have the great concept of Light Armor + Daedric Weapons when they begin, which is perfectly possible in Oblivion