I don't think null magic mantle should be a starting item. It obviously caused a LOT of problems and shut down some magic damage champions too well. That being said, I REALLY hate the argument you are using to justify the decision.
Yes, auto attacks are on a shorter cooldown. Yes, champions start with ad. At level 5, master yi sits at 69 ad before runes and masteries. That is certainly more damage than the AP a mage gets, right?. But you completely disregard the fact that a mage is putting levels in abilities; and the base damage on their skills ramps up MUCH faster than ad champions; at least until the first back (maybe). At level 5, malzahar E is up to 200 base damage. Vlad Q is up to 160. Annie Q is up to 150. Yes, these require mana (sometimes). But they are hitting far harder than an auto attacker could possibly hope to match up against.
Of course, there are some ad casters (like riven) who have good physical damage abilities as well. But if your argument is going to compare spells to auto attacks, the statement that auto attacks are better is only true after around 10 minutes.
So in addition to the fact that abilities deal much better damage, are much safer to use, scale much better (at the very least they scale better before the first back), and mages can itemize against AD champions from the start of the game, I don't think this argument is particularly fair. It gets even more unfair when you consider that spells never proc minion aggro while all auto attacks do. Bottom line here: it isn't just fair to compare auto attacks and a spell in a vacuum like that; there is so much more going on in the early game that this argument is rather poor.