So, in your mind, professionalism is more important than integrity? Most actors most likely do lack integrity. But mostly out of greed, pride (they have rather large ego's) and the fear that speaking out will cost them their career.
Mark Hamill had/has the voice and the hearts of the fans. KK & Johnson basically told him to shut the f up when he voiced his opinion about this "Luke" character and the direction they had him going in, and that's not what the fans would want. They simply treated him as an actor, not someone to take seriously or give creative input. He seems to care more about the SW franchise than any one else at Disney. That in itself is sad. I't wasn't just a paycheck to him like it is with most actors. Specifically Harrison Ford who has always been vocal about his character. I think he was politely saying "I told you so", as he well knew there would be ample backlash towards TLJ.
So, what you say Mark Hamill lacked in Professionalism, I say he made up for in spades with Integrity.