Similarly, if Android phones lose headphones jacks and expandable storage, there's no reason to stay on the platform anymore and be the outsider in the iMessage world when the hardware is otherwise equal these days.
People have, but changes have been made where the barrier to entry on the crypto/client verification side is sufficiently high a barrier to make it not practical. See next answer for more context.
> Is Apple using any hardware stuff to prevent this?
Yes.
> I think an unofficial iMessage client appeared a few years ago, but it was very closed source and I don't think it worked for very long
The iMessage team scrambled on its release to block it as it leveraged a severe security breach.