Every time I plan a trip with my friends, someone drops out, the dates need adjusting, and the trip ends up looking different from the initial idea. But that's okay; it can still be a great trip. What I find unfair is telling someone they're no longer invited after they've paid a hotel deposit, and then even expecting them to cover the remaining balance.
This is what happened to a boyfriend who posted his story on the Reddit AITApod subreddit. He couldn't get an extra day off approved, so his girlfriend disinvited him from the trip.
When I went to Oktoberfest with my girlfriend, it was originally supposed to be an outing with friends. Then all our friends dropped out little by little, and it ended up being just the two of us. We adjusted the plan and still got to go. Our friends' outing turned into a couples' trip, but we had a great time all the same. The good thing was that no one had paid for anything at that moment, so the change was easy to handle.
The fact that we hadn't paid yet changes the story. Once people start paying, changing the guest list when it involves paid reservations becomes more difficult to manage.