I don't mind if you make him ranged. My problem is he's not a true ranged champion. He's almost a worst of both worlds ranged/melee hybrid. Pros and cons of his current AA's:
Pro
+Slightly longer range attacks than a standard melee champ
Con
-Less base AD than a melee champ
-Is expected to build as a tank support, but can't proc the gold generation trait of the tank support item (minion execute).
-This means you either take relic and risk pissing of your ADC if you can't last hit (albeit E passive helps a bit with that), take spell thief's and get raged at for trolling (harder to proc that item too due to my last bullet in this list), or you take ancient coin. If you take coin, it's bad enough that you already have to chase around souls, but now you have to chase around gold drops too since coin no longer automatically collects gold from every nearby minion kill.
-Lowest range of any AA champ
-ONLY "ranged" champ with NO projectile. Every other ranged champ in the game shoots/throws a projectile, meaning that at some point in the attack animation, the projectile leaves the champ, and (s)he can attack move to cancel the rest of the animation. This is mostly used for kiting, but Thresh can't do it. He needs to complete his entire attack animation for the hit to count, and at no point can he move out of his animation without completely canceling the AA. Half the reason for a champ to be ranged is to be able to kite melee champs.