They could be fined and then forced to pay full whack.

But I'm not sure they meet the threshold for anti-competative practice. You'd need to show some sort of paid exclusivity (eg a bribe so only Spotify could get a better rate).

30% for market access is a bigger net evil that anything Spotify might be doing IMO.

They run very slim profit margins, regulators could fine them more than they could afford to pay.