Except both of their damage is too inconsistent to justify the lack of mobility.
You are threatened by ganks, mobile characters are not. They can wipe one person off the face of the planet, wait 7 seconds an drop another to half. Not to mention both have better CC in a consistency fashion. Leblanc has a slow, bind that relies on nothing else but hitting it.
Brand needs a marked target for his stun, and no way to make the shot easier. The stun is nothing spectacular either compared to other stuns in the game, nor is the damage. Compare it to Ahri's charm for example.
Heimer's has a giant tell and is much harder to hit in the sense that unlike a normal skill shot, it doesn't hit them if they dash towards it or run away. You have to predict for the exact place they would be at.
Throw on that if they run away you are just boned and can't follow where as if you get engaged on you are just screwed unless you can 100-0 all of them.
The only character (imo) who has the burst, damage and lane control that is immobile and feels good to play (again, imo) is Ori cause she has a ton of utility on top of mass burst potential only reliant on her ball. but her ult has so much damage and CC that is immidiate, unlike Brand, Heimer, Victor(Victor does and doesn't, His CC is better then most imo but still not worth playing him over other options, and at least he has a movement speed increase and a good slow stun). Cass got number buffed until she was good even though I don't know anyone who "enjoys" playing her more then she is "strong now"(This is friends, not random people on the forum. Since its a better stat since smurf accounts and so forth).
Veigar is getting changed in ways I dislike personally. Making him dramatically harder to play with less trade off. Which goes back to why Leblanc and Kass are just better, is because they can pick a target, and that target is probably screwed if you can't rip them off instantly. Same for Zed really. But Veigar no longer has the ability to pick a target and they are dead. No instant stun to give his dark matter time to land. No targeted q so ignore the front line to pick a high priority target. No DFG ult combo. And anyone who can stack better with the new q(being a skill shot it will inherently be harder to stack, anyone who says other is assuming theoretically perfection that should be possible against any player of equal skill and since any good player should punish you for stacking it) probably had to put dozens of hours into play. And all for what? The double hit q, better mana costs(which were ruined by the mana changes anyway and is more ofa fix for that) late game cooldown reduction(cause the game was hard cause of his late, not his early. So we made his early game worse) , and a debuff on his ult.
Veigar might be good. I suppose with his skill ceiling higher there is the potential for him to do better. But honestly? Where is the fun in picking him? Again, I haven't played him. But I'm the type of person who ran the numbers on elise and knew she was good top when she was released. Who knew the Cass rework was going to feel utterly awful with the numbers on that and the mechanic changes. I like running numbers and running scenarios.
And in all scenarios where counter play is added to characters with no mobility, characters with mobility become more unenjoyable to play against.
I still come to these forums hoping to see changes coming that will make my favorite characters(zero mobility characters, including supports, and tanks) more fun to play, or more viable or give mobility characters weakness that matters. And Kass getting his ult changed is a start, but after 3 years its hard for me to care with just that.
Also Veigar getting played in the LCS after all this discussion on him felt timed so stupidly perfect.
Also to add on the fact Kass and LB win statistically less over the years, wasn't that due to Kass and LB both being so bad for a year and a half nobody touched them? Also heimer didn't have that high of a win percentage before his rework did he? Brand maybe, I don't know, but he's always been unfun to me because his ult just kinda goes where it feels like and his stun doesn't feel good or his kit. If zero mobility mages as a rule could cast while movinng, I might be less of that opinion.