I seem to remember something like that was discussed at some point in Signal Community and one of the big arguments contra was that the normies and even some advanced people who use Signal just don't "get" UX warnings any more: you could have put all the red tabs, cross signs and unlocked padlocks on the screen you wanted, they were still gonna complain openly on the internet and discredit Signal for not actually "securing muh messages". It's actually part of the same argument why they don't let you export your own messages.

Besides, having to use the same engine (tab and all) in the same app for two things with vastly different reaches of security was murder on the dev team. There were things they were not able to keep testing because of clashing against SMS compat. It is one of the reasons why I think the smart thing to have done would have to keep the "original" app as the Signal SMS.