I think the issue is that fans are loyal to a club, but players are only loyal so far, even with Trent being at the club since being 6 years old. That's the thing, that as fans we can't get our heads around and forget it's not necessarily the same way for players.
After doing everything and experiencing everything, even the most ardent employee, which the players are, would get itchy feet and that's at the point where Trent is in his life. He has the opportunity for him and his family to go do something new, which essentially he's not done in 20 years. Yeah, it would have been great for him to sign a contract and still be sold, so we got a transfer fee of some sort, but from his perspective he wanted to ensure this move happens.
Time will tell, as the grass isn't always greener, but it's his choice and he's out of contract and not beholden to the club or the fans, no matter what they think.