the spell itself being powerful is fine, but late game even if he isn't a carry any more wither still might as well be a disable for any auto attacking champion he's in range of.
imo the biggest thing that gates the rest of him is the fact that wither has absurdly low downtime. with 45 cdr you have 1 second before either comes back up which is insane for being targeted and with no other conditions to it. the sheer power this spell has for its reliability is nuts and guarantees he'll at least have SOME relevance in any game
i think weakening wither might be necessary in order to free him up for power elsewhere, or by giving him a condition to fulfill in combat like most other juggernauts have. some quick ideas would be making wither have an initial tether on it or making it stronger when the target is on his E. these are just spitballs but I think adding a little more complexity to his spells would be a good start to modernizing him