It is a great app, but you cannot fit everyone into a single app.

Examples why I personally sometimes don't want to use Signal:

• no native desktop app, just a half-baked Electron based thing
• no versions for systems other than Android and iOS
• requires phone number (common argument)
• hard to integrate bots, notifications and automatic services for the future use
• when Signal foundation do something stupid, it would mean me having to migrate all friends yet another time

Signal is super giga great, the cons list is short, but if we want everyone to use something it has to be an universal protocol, not one app.