It's not about whether big tech companies — or any companies — cause "market failure". It's whether their behavior is anti-competitive. Are they leveraging their market leadership in a way that hinders other players from participating in the market? Not just, are they making a better product, or have more users: are they interfering with other companies efforts to compete?

How that question is answered regarding AmEx will shape how future similar cases, many of which will involve tech, are judged. There's no equation between them; it's just what this decision will mean.