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.
I love that "there are two big players in the market with a literally 50%/50% split" means that iOS dominates. iPhones have always been popular among teenagers, if Millennials are any indication it levels out over time. It's just one of many weird demographic splits, not some grand trend. People need to not fall into the fallacy that "something is popular with young people" means that thing will remain popular as they get older.
iOS is more popular among women, young people, liberals, professionals, upper-middle and high-income, people with post-secondary education, and urbanites.
As well as the inverse of the above Android is more popular with people who work in IT, and people who follow tech news.