That's true. But my impression is that he has a lot of options. I would also guess they paid him more than most of the main characters put together. I could see him coming back for guest spots, if he really did enjoy the experience, but I assume part of the appeal was that it was a limited-time engagement. If he'd wanted to sign up permanently, and they wanted to pay him to stay, I'd guess they could and would have made that happen.
If I were him, I wouldn't want to play a more milquetoast version of the same character. If they can come up with something super-compelling for ReguLorca, maybe they can lure him back. But so far there's only been one character like that, and it was in no small part because Isaacs was playing him.