Why is Zilean only considered a support?

Both on here and reddit, the general consensus is that Zilean was reworked into a support and his solo lane is gone.And he kind of is a support. While ARAM clearly shows he has annoying damage, he has great utility and since his lane isn't that great, he is better off down bot.

But I don't understand why people insist that Mid Zilean is DEAD. I have a funny feeling that if LCS ever took him mid, you would see him in mid again by all the lcs meta sheep.

Honestly, his early game is brutal because he is dependant on blue buffs to waveclear. But once he has Athene's, it's off to assist city. His bombs still hurt, but honestly even if he is a little behind, he still packs wicked utility. The speed, slows, stun and ult can provide so much for a team even when behind, moreso than a starved assassin could ever provide. Being fed just makes it worse.

Mid game he is perma sped, being able to roam wherever, whenever, doing whatever it is that needs doing. He can quickly go to drag and make plays.

Now that Kindred is a thing, nothing is more hilarious than just dropping a bomb on her and speeding off, letting her be content in her death-free zone just to get blown up by the delayed explosion.

I won't lie, it requires a lot of finesse and you really need to know what you are doing, you really can't facemash to win like some other champs, but it can work. And when it does, it catches people off guard because "ZILEAN MID!??! WTF THATS NOT A THING" All his damage may be tied to a delayed skillshot, but Xerath and Vel'Koz all have telegraphed, non-guaranteed damage too. Even with the likes of Fizz, you'd think he would counter Zilean because of his E, but it's that counter that actually counters itself. If Fizz wants to dodge your damage, he has to use his only escape. And when you build AP, it certainly hurts and it is very tempting to Playful/Trickster, meaning he won't ever be able to safely dive you without getting double bombed.