First, not every iPhone user can be iMessage-only and SMS is a shitty experience. In most non-US countries, they’re not winning the iMessage shame battle, they’re driving people to WhatsApp. They’d probably rather people use the built in messaging app, which will incentivize you to use iMessage.
Secondly, getting more involved gives them opportunities to embrace/extinguish rcs (they said they won’t extend it). If they can use their market to force encryption on everyone, for example, that makes them look like the good guys against android makers. I could also see them trying to genuinely push UX improvements (eg stickers) on RCS and exiting iMessage from European Markets entirely.
Finally, If the EU forces open iMessage, it’ll probably force open other messaging services, which gives RCS an opportunity to grow. See point 1 about them losing already in Europe. Or gives Apple a horse in the race even if they simply withdraw iMessage instead of opening it up.