It's extremely expensive outside the US and can't be sent over wifi - so if you're communicating with someone abroad it's not a very convenient option. You're also missing out on E2EE and, lastly, Apple has corrupted the utility of it as a communication method for half of the devices out there.
I'm not in US, and unlimited sms is included in monthly subscription.
And years before that sms was cheaper than data.
Also sms is cheaper when roaming in Europe (in my case it has zero cost besides the monthly subscription price).
Sure IM are etter if you need group chat or communication abroad. But that is not the case for majority of population where 1 to 1 communication is used inside a single country.