It’s a valid point that it could potentially create some confusion when a user assumes that everything in Signal is secure. Unencrypted SMS threads could contain an open padlock icon and even an ominous red window border, but someone inevitably will not understand the difference.
However, my frustration has been how both convenience and security is reduced by removing SMS from Signal.
Many people will continue to use SMS for a variety of reasons, necessitating the use of an additional app. So now we have people continuing to communicate over this insecure protocol, but with the additional target vector of potential vulnerabilities in the supplemental app.