All of that happens only if the user chooses to do it though. Anybody is free to stay in the caged Apple garden. The EU just wants them to leave the door unlocked.
A door with a lock is different from a wall with no door. Same argument that gets made with government-keyed or government-breakable encryption schemes: it's better for everyone to not have the backdoor at all.
That's flatly not true. imessage interop means not just the person who installs the other app, but the data of everyone with whom they message loses the security/privacy guarantees created by imessage and Apple as a corp. Including massive resources pointed at securing the app itself.
That doesn't necessarily mean it's a bad idea from a competition point of view, but good ideas can be discussed w/ an honest view of the quite real downsides.
Apple could have worked with other companies to make RCS secure by default instead of building their own little thing that openly and intentionally discriminates non-members of their club.