Could Veigar's E Stop Dashes Immediately On Cast Even Before The Full Cage Is Up?

To clarify, it wouldn't stun anyone until the cage is actually up but there's nothing more frustrating as a Veigar player then using your E to escape a gank or all-in from the other laner and they dash out of it to you before it's up, making it useless. Then you have no defense at all. Yes you can stand on the wall and hope they run into it to attack you but most people aren't that stupid. Having it stop dashes immediately on cast would help Veigar's defense considerably. Considering he's completely immobile I don't think it's too much to ask for a more reliable form of defense against high mobility characters.

Champions could still dash to the edge and walk out so i feel this would be fair. It would just stop them from jumping on Veigar immediately making it more fair for him instead of his E being useless.