You're not worried that adding a real passive adds to an already really complete kit with few weaknesses?
I think if his passive is good, this helps bring up his low end a lot. By definition, he has pretty low agency over how the Sun Disk performs, so I believe it'll tend to carry more weight in Bronze than in Challenger.
Now, this almost certainly can't carry the entire burden. Goal #1 is being a fun champion. But right now the passive is really ultra horrible to use and that can get a lot better. So pretty much anything is better than the current state, even if it eats up a little bit of his power budget. Since again, his power level is completely pro constrained and pros are REALLY good at playing around towers, a buff here likely doesn't move the needle a whole lot in pro.