As a user, I do want an alternative to Apple's Messages app (such as Beeper Cloud or a standalone SMS/MMS client with additional features) that can handle SMS/MMS messaging on iOS. Your choice to use Apple's Messages app does not justify Apple's anticompetitive measures to ban competing SMS/MMS apps on iOS for me or anyone else. If you don't want to switch to a non-Apple SMS/MMS app, then simply don't do it.
It's common sense that when Apple forbids competitors from implementing a feature that they use for their own product, competitors are harmed. Apple previously made Apple Mail the default email client on iOS with no option to change it. After Apple removed this anticompetitive restriction in iOS 14, competing apps such as Proton Mail and Tuta (which used their own protocols instead of the default IMAP/SMTP protocol to support end-to-end encryption) immediately became more convenient to use on iOS.
The issue with Apple's SMS/MMS client restriction is more severe. Not only am I not able to designate a different messenger as the default SMS/MMS app on iOS, I cannot install one in the first place because Apple has banned them altogether.