Victor French did have other things to do, and for a while, they were not speaking, so that seemed like an easy thing to say, they could always bring him back.
I know the actor who played John, was disappointed that they wrote him off later and this seemed like the beginning of that. If he wasn't going to have a relationship with Mary, there wouldn't be any use for him. I know some were saddened by how they made him a louse, but it wasn't that unbelievable. He was there for a while, and became what was around him. He never would have wanted to move back to Walnut Grove and I couldn't picture Mary in that lifestyle. His lying was bad, leading her on, but he's not the first male or female to have trouble saying "It's over", although John had a way with words. He just kept putting it off. I felt sorry for her seeming so out of place and the shirt she made John seeming so out of place also.
I know it fits the plot, but they always made "city" people overall bad, the girls were not as proper, too fancy and the men, rude and not very Christian. I'm sure they had big differences in the atmosphere, but I'm sure nice couples and young men and women existed.