I think you're absolutely right that it's a complex issue. People hoard for different reasons and some people are more reachable than others. However, I worry that some people seem to think that it's just hopeless so why bother trying. I can only speak for what I have seen work: Patience, persistence, respect, and positive reinforcement

Patience in that you don't want to show up one day and announce you're clearing out the house. Small improvements to specific rooms over time. You might even just start off with a single closet.

Persistence in that you don't give up after being rebuffed a time or two. Realize that it will likely be a lifelong struggle with this person, but it can be managed.

Respect. I've learned that you really have to respect the fact this junk is their junk. You wont get anywhere throwing out stuff against their will. Ultimately they have to be the one to agree that it's getting junked. I found it best to start off with low hanging fruit. Start off with broken, or expired stuff. Then getting them to trash things they have copies of. Be willing to compromise.

Positive Reinforcement. Try to avoid being too confrontational throughout this process. If you do clean something out compliment them how nice the newly cleaned room/closet/fridge is. Compliment them if they tell you they threw some stuff out or cleaned something all by themselves.