[quanticle]

    The only reason Circle isn't bothering with this market is seemingly that they didn't want to deal directly with retail

That's actually a really good reason! An analogous situation is with e.g. Robinhood and Citadel. In one sense Robinhood is "just" a retail onramp. All their trades go through Citadel. So in theory Citadel could, at any moment, just cut off Robinhood by allowing retail investors to trade directly with them.

But I'm willing to bet that they won't, because dealing with retail investors is a hassle. You have to have customer service. You have to deal with additional regulation. You have to deal with random social media firestorms. It's a mess. I don't blame Citadel for leaving it to Robinhood to deal with these issues and I wouldn't blame Circle for leaving it to Coinbase to deal with the retail issues surrounding crypto.