Quick answer. Use SMS instead of RCS when universal reach and simplicity matter most: messages that must reach every possible device regardless of RCS support, plain one-time passcodes and critical alerts where a single line of text is enough, and recipients who aren't RCS-capable. SMS is also the automatic fallback for RCS, so in practice you don't pick SMS over RCS — you send RCS-first and SMS catches everything that can't receive rich messages. Pure plain-text, reach-everything use cases are where SMS still leads.
There's rarely a reason to deliberately downgrade an RCS-capable recipient to SMS. The real role of SMS in a modern program is universal coverage and guaranteed delivery — the safety net beneath RCS, not a competing choice.