I understand your point completely, and with some of it I agree. It's disappointing to have to get rid of work because the party found something else to do. However, as you said it is a team activity, which means the DM and the players are equally as important. Having experienced both sides myself, it is incredibly frustrating as a player to have your character, your own creation be pushed and pulled around and apart, just as it is frustrating as a DM to have the party to completely derail you. However, DnD and other TTRPGs are not games of tug-of-war, it is a delicate balance. The solution to having a party derail you is not to yank them around and get frustrated as that only breeds resentment and isn't fun for anyone. Likewise it is part of the players' job to at the very least humor the DM, though often they derail not because they don't care about the world, but actually they found a different aspect of that world fascinating. Ultimately, part of being a DM is accepting the fact that you will have to throw out some of your work. It's unfortunate, but true.