Eidolons like Rudra are highly desired due to how much they raise your theoretical ceiling combined with their rarity. 100%+ eidolons are the best thing to get for people rolling new accounts; no question about that. They're also the best individual pieces to get. But they're still just individual pieces of the puzzle. Conversation about the high end usually implies access to both 100%+ eidolons and SSR kamihime.

So yes, maybe one day you get a 100%+ eidolon in another element. Or maybe you roll a couple of stronger SSRs in another element. Or maybe even your SR rolls are tilted in favour of other elements over water. Then, depending on content, you face the question of, will your Rudra + SRs team outperform this other team that has element advantage? You're not just overcoming the gap in damage output in a vacuum. You also have to compensate for the difference in damage you're taking back through a combination of killing even faster and/or having more damage mitigation tricks.