The US Department of Justice and 16 state and district attorneys general accused Apple of operating an illegal monopoly in the smartphone market in a new antitrust lawsuit. The DOJ and states are accusing Apple of driving up prices for consumers and developers at the expense of making users more reliant on its iPhones.
The proof is the status quo. Video texts from Android users look bad on an iPhone. Apple could choose to fall back to RCS instead of SMS from iMessage. RCS would offer better video quality than SMS, which overall improves the interoperability of all phones. Because RCS is a standard and the natural successor to SMS, refusing to support the standard makes it less likely to succeed, with the intent of defending their dominant market share.
While I agree with you, this isn’t as outright as I though it would be though. Apple fan boys could very easily just handwave this away. Frankly I don’t live in the US so no one here uses iMessage anyways so I don’t really have any examples I have seen or could use to show people.