At some point in the story Oda decided to give Roger a narrative "protagonist" status.
Roger is the protagonist of his era and Oda falls back to this era quite frequently, especially post timeskip.
Because of that it makes sense to have Rocks be this strong, so we can follow the struggles of the old era against Rocks.
Rocks will also serve as an inspiration for one of the main antagonists/main rivals in the series.
It kinda makes sense to me, even if he is dead.