I support Signal's decision to drop support for SMS and MMS. Software maintenance can be incredibly challenging and time-consuming. This decision will likely free up time for more important work.
John Carmack is rumored to have said, “Focus is a matter of deciding what things you're not going to do.” I'm not sure if the attribution is correct, but it doesn't matter. It's a good point.
Contrary to the opinions shared on Hacker News, the world is not going to end. (Hacker News readers often forget that they are not the target market.) If anything, it might be easier to convince others to use Signal now. “Use this app to have private conversations with other people who use the app. It doesn't change how anything else on your phone works.” In a world that remembers rouge software crashing computers, that fact is more important than it might seem.
Besides, abbreviations rarely correlate with usability. Signal needs to reach normal people. Let's keep it simple.