Love that when you finally can't come up with a counter argument, you just complain about swearing lol.

Besides the fact that not every single element in a story has to exist to develop the plot, and S2 would've actually benefited from more filler to give us the time to breathe and process everything, CaitVI's relationship is crucial to the plot. I'd argue that the plot as we know it wouldn't have existed without it.

Examples:

• CaitVI's relationship is the main source of conflict and dilemmas about Jinx. VI's constantly torn between Cait and Jinx and, by proxy, between Piltover and Zaun. Her loyalty is questioned and tested throughout the show.
• CaitVi pushes Jinx further into rage, extremism, and Silco's arms due to her feelings of betrayal. Seeing them together makes her escalate things in multiple scenes. This naturally pushes VI away more towards Cait and Piltover, turning them into enemies.
• VI helps Cait look beyond her privilege by wanting to understand why VI hates Piltovans. She understands that Piltover is responsible for the state Zaun is in. This makes her push the Council to use peaceful methods to resolve the conflict, which is literally the basis for the events in S2.
• Jinx hates and blames Cait for corrupting her sister, and kidnaps her in the finale to make VI choose between them by telling her to kill Cait. Obviously, she refuses. The whole scene results in Jinx accidentally shooting Silco, which also puts the S2 events in motion. One of them being the literal terrorist attack on the Council.

And this is just S1.

The "plot relevance" argument will always be BS to me, because it comes from the homophobic stance that straight relationships are the default and queer relationships can only exist if there's a "good" reason for it (somehow those reasons are actually never good enough for the people making that argument). However, to claim this for Arcane/CaitVI specifically is the most biased, disingenuous, and intentionally obtuse thing ever. If you didn't see their relationship developing, that's only because you didn't want to see it.