Objects earn space through use, meaning, or beauty; everything else should explain itself.

Three things justify keeping something: you use it, it means something to you, or it makes the room better to be in. Those are generous criteria, and most of what accumulates meets none of them. It stayed because leaving required a decision and staying did not.

Run a shelf through those three questions and notice how quickly the answers come. Anything that needs a paragraph of justification is probably not earning its space. You are not being ruthless, you are just making the default require a reason.