I like that you accept the wholly arbitrary decision of the AD&D designers and pretend it is a reason but in DAi you can't.
I doubt there is a codex entry that says you can cast all the spells you learn and not some subset. Maybe I'm wrong but then again I don't even care if there is. Gameplay > lore. If the designers think gameplay is going improve with a minor, and yes this is minor, change in lore....so be it. They should not be slaves to something written and scantly read by most players half a decade ago.
Of course you will have level based limitations. No rpg just gives you every spell out there at level 1. So yeah...limitations....
And I am not pretending there is a reason. There is a reason. In D&D it is tied to your intelligence stat. It is at the very core of the rulesystem.
Can you give me a good in-game reason why a mage can't use all his spells in a fight in DA:I? He just forgets the spell? He decides not to use a particular one, even if it kills him? If the inquisitor is a mage, and he dies from this, the world ends. How dumb would that be? dooming the world because he refused to cast a spell he already knows.
The magic system is not a minor thing. In a fantasy world where magic is pretty much its science, it is a pretty damn big thing.
Stunning argument. Some people didn't read the codex, so Bioware should ignore it. Yeah. Smart move. That effectively makes the whole setting fall apart, and it has no believability left. What would you change next? Gravity? Nobody reads about gravity in the game. Lets get rid of it.