I asked Signal motivations for SMS removal
I think you're forgetting the main reason: the group of people using it to communicate is really small and shrinking every year.The only large group of people who still primarily use SMS to communicate person-to-person is Android users in the USA.Every other country has settled on either Telegram, WeChat, WhatsApp or FB Messenger, or other niche apps. These apps work on both iOS and Android and often also Windows. I haven't sent an SMS in probably 12 years. I don't know anyone who has.It's only in the US that iMessage is so prevalent that Android users have to use SMS, the only other way of messaging iOS devices. And the US is quickly becoming a de-facto iOS only country. It already has more than 50% market share, even 80% among young people.With the US going (almost) full iMessage and the rest of the world having already settled on another app there simply no point to supporting SMS.