So you wanna nerf a midget who has no way of defending himself other than a kind of telegraphed circular stun? I would've understand it if the cage stunned whoever was on the inside too,but if the veigar you are facing isn't an OTP or some like that,you will always have a way of dodging his skills,in his cage too. The only thing he does not have to aim it's his ultimate,which since patch 6.9 it's an EXECUTION,so as long as you don't get hit by his other spells,it will deal much less damage.
But seriously,he himself only has DAMAGE as it's way of playing. Or you play it tank using it's infinite scaling,but not many do that. He also gets countered by a lot of people,more famous Lux;Fizz;Vlad.
And by the way: most of the "Jumps" and dashes get cancelled by the cage,you have to use blinks,like ekko's 2° cast of E,or Fizz's E,or Ezreal's E. Tristana's Q or Shen's E or Akali's and Diana's ultimates would all get cancelled,unless their skills says otherwise (Shyvana's and Malphite's ult)
If you're not the midlaner losing against him,you can win by camping his slow ass. He seriously has NO MOBILITY and low movement speed,so if you can dodge his stun,he is pretty much dead unless he is constantly under tower. And in that case,he may have cs or mana problems (due to having to cs with Q to get powerful quickly)