This would of course be different if Apple prevented you from sending SMS messages via iMessage, or from installing third party messengers. But they don’t do either of these things.
Apple does, in fact, prevent you from installing a third-party messenger on iOS that uses the device's native SMS/MMS capabilities. Apple's own Messages app is the only app that is allowed by Apple to handle SMS/MMS on iOS.
The fact that Apple has made iMessage exclusive to Messages, paired with the Messages app's privileged position as the only SMS/MMS client Apple allows on iOS, gives the Messages app and iMessage an unfair advantage over competing messengers and messaging protocols.
I don’t think a competing SMS/MMS application would shift the competitive landscape significantly here. If it was, we’d see Beeper making an SMS app and challenging Apple’s inclusion policies there, not reverse engineering iMessage.
As a Signal user, you might have remembered when Signal/TextSecure supported SMS/MMS on Android in addition to Signal Protocol messages, which was a big factor in helping Signal onboard new Android users when it had a smaller user base. Signal never had the opportunity to do the same on iOS because Apple only allows its own Messages client to handle on-device SMS/MMS messages, which gave Messages (and iMessage) an unfair advantage over Signal (and the Signal Protocol).
Anticompetitive measures like Apple's SMS/MMS client restriction harm the market even when alternatives to Apple's products exist. The entire blue vs. green bubble issue would not be a problem in the first place had iOS users been able to switch to a competing messenger app that supports both SMS/MMS messaging and a newer cross-platform messaging protocol (instead of iMessage).
Also, google voice still doesnt have rcs by the way, and it’s safe to assume that they will finally sunset the service rather than follow through on “principled stand” around rcs lol. Much like the epic games case… there was never anything more than a ploy to look good to regulators/lawyers. People are tragically unable to identify even very blatant cases of special pleading, especially when they align with the anti-apple zeitgeist among large parts of the tech crowd.
So opening the standard is great. The question is how will it work and how will the other parties guarantee the level of service. How do they deal with spam for example? Do they build their own network or do they use Apple’s? If Apple is forced to open up does it mean they’re forced to give access to any of their infrastructure? Can they charge or will there be a price cap? Who drives the evolution of the standard?
The details matter more than the principle in this case because on principle you could just end up giving everyone a crappier service.
(Separately: I don't think Apple's concern is about level of service. I think they're -- reasonably -- worried that the security properties of their messaging system are harder to uphold without confidence in their client itself. This is not an unreasonable concern.)
But again: a network effect is not evidence of monopolistic or anticompetitive behavior. Twitter has (had?) a network effect, and could not meaningfully be said to have a monopoly over social media.