I can understand why he did that, but I kind of get D&D's point too.
I mean, for one Ian is a huge fan of the books and was well aware of what was to come, he had some strong story coming up. He's obviously passionate enough about it that he was disappointed he wouldn't get into the meaty part of the story and confused maybe wanted to make sure D&D weren't missing the point and an opportunity.
On the other hand, he is just a minorish actor on the show and has no idea of the long term plans the writers have. Getting a letter like that could be seen as in insult to them. It's not really his place to try to dictate the story. It's like a temp worker asking a CEO "are you sure you know what you're doing? Why don't you try this instead, it's better."
Really it comes down to the letter itself I guess, I don't see Ian being confrontational or rude about it though.
I do think they were only half joking when they said it made them want to fire him even more, they don't seem to like when people question their choices anymore.
I will forever be pissed we never got the Khrazz fight though. Barry's last scene was ok because he took out so many guys on his own, but it didn't look overly impressive.
They should have at least changed the scene some how.
Have a throwaway scene in a previous episode with "the most dangerous pit fighter" taking to somebody sketchy about overthrowing Dany to establish this guy.
Then have him bump into Barry the Bold while in full armor. Ian puts on his helmet and is swapped out with the guy who played Dayne.
Epic fight goes down in the same alley but it's like in the books. After a full minute of stellar choreography which makes it clear Barry is loving it he wins, obviously. Threat averted.
But then from every fucking corner 30 hizdars come running out in their masks. Barry starts casting them down left and right like nothing. There are too many, for every 1 he kills another 3 come running in. They eventually just dog pile him basically and he is no loner visible. We see Barry's sword pierce through the chest of one of the bodies but then the pile collapses as Barry dies.
Hell the way I just played that out in my head is my new cannon for Barry's demise.