There is no U.S. antitrust provision that would force Apple to place a backdoor in iMessage. And if iMessage is compromised in certain countries where secure messaging apps are banned (such as China: https://www.reuters.com/article/us-china-apple-icloud-insigh...), iOS users would benefit from having the option to sideload apps like Signal and use them to communicate securely.
Forbidding one company from taking anti-competitive actions against smaller companies that increase the prices its users pay for products and services is not taking away from your rights. If you don't want to download or purchase anything through means not endorsed by Apple, you're not being forced to do so. The App Store and Apple's payment system are still available to you. The idea that others should conform to and pay for your preferences is antithetical to the notion of freedom.