More than 10% of europeans use iMessage, enough for EU Digital Market Act to force Apple to adopt RCS and thereby trying to circumvent opening iMessage itself
Apple won't make the iPhone fall back to sending RCS though Google servers (like Android does) if there isn't carrier support for it, they'll just say "there is no RCS support on your carrier" and go for SMS.
iPads can do iMessage. iPhones with no phone plan can do iMessage. iPod Touches, used to be the cheap messaging devices for kids before phone plans became more widespread. Macbooks. Those cannot send or recieve SMS/RCS.