My take on William/TMIB - William was thought by Logan to be some kind of "doormat" type personality who was wimpy and ultra-conservative in conduct as that is the way he had presented himself to the family and his bride to be. He discovers that William is really the opposite and is really ruthless and violent by nature (being in the park allowed him to reveal that side of himself.) Whether Logan is alive or dead, the narrative is firmly set in the family's mind, that Logan is a playboy while William is a stable, hard-working type who can be trusted with the family fortune while Logan would drink it away and endanger it by his impulsive and reckless behavior. Thus, William ends out.
At William's behest, the DELOS family comes to acquire controlling interest in the park and William takes numerous treks there trying to figure out the "back-story" (Maze). While doing this, after a while it all becomes boring to him as he realizes that he always wins, the hosts always lose.* He's interacted with a number of the hosts for that 35 yr time period, but none more interesting to him than Doloros and it hurt him much more than he allowed to express himself about it, when Doloros had no memory of him on his first return trip. Thus setting up his quest to discover more about the workings of the hosts and the potential for them having actually "consciousness" and perhaps to have things changed to more "real life" conditions where the hosts would at least have a chance of "winning" while the human guests would actually be betting with their lives/health.
*This reminded me of the classic Twilight Zone episode involving a low grade gangster who dies in a shoot out with police. He arrives in the afterlife and is met by an "angel" who shows him fancy new apartment, his harem of girlfriends, the casino where he can never lose, etc.... In the end, he comes to realize that because he can never be denied what he wants, he never fails, he never truly risks anything, his life is so boring and devoid of any kind of actual excitement he can't believe that this is heaven. That's when the "angel" informs him he's not in heaven, he's in the other place.
William is so bored out of his gourd, he's been trying to get Ford to actually make things more interesting by allowing the hosts to have a chance. That's why the big smile on his face at the end, his wish has come true. (Besides, what did he really have to live for at that point? His wife had died, his family disowned him, and he discovered that the "secret of the maze" really had no relevance to him whatsoever, but was only something the hosts understood.)