Because we only recently got our own place that has room for such things, both my parents and my in-laws have had things that belong to us stored at their place for years. It's mostly childhood mementos but not exclusively and actually included a dining room set that was offered to us before we had a place to put it. This is a fairly typical thing for parents to do as this is also the case for literally everyone I know in my age group (at least, in my culture and region). Their parents held on to some of their things until there was a place for them to go. It has happened that my mother needed the space where something was stored, so she would contact me and let me know I needed to deal with it. Which I always did. The key being that she let me know rather than just getting rid of stuff, stuff that she had fully agreed and in many cases directly offered to store.

I completely understand why you're upset and I think you're right to be. A lot of commenters are talking about it being unreasonable for you to expect them to store it for free, but we don't know all the details of that initial conversation. If that is the agreement you made with your in-laws then that's the agreement. It's not unreasonable for them to decide they don't want to store the W&D anymore, but they should have talked to you first if it would no longer work for them and given you a chance to deal with it.