[brokenodometer]

Substitution and lock-in are also things that the antitrust laws are concerned with as they are relevant to how you define the market in the first place. If the market is for iOS browser engines, Android browsers cannot effectively compete for consumers who are locked-in to the iOS ecosystem due to the switching costs and other barriers that must be surmounted in order for consumers to access those browsers. Almost no one is going to switch from an existing $1000+ phone to a new one solely because of their browser, leading to almost zero cross elasticity of demand between iOS and Android browsers (high cross-elasticity being indicia that products are part of the same market, e.g., mobile browsers).

So one way to answer your question would be to say that Apple has complete control over the relevant market, which is the market for iOS browser engines.

[irrational]

But, nobody pays for browsers (anymore). What are they competing for? It is ALWAYS about money. Are they competing to see who can collect your data to see to third parties? Who can show you ads in their browser? Those are both bad things in my opinion, so Apple blocking them is consumer friendly behavior. But, of course, in a country ruled by corporations, this would be seen as bad.