Great metaphor, thanks!
However Marginal Utility theory describes only rational behaviour. It is far from certain that such behaviour is characteristic to majority of people because deviations are very common.
That's because "wants" are often disconnected from reality and limited only by ability to acquire stuff. It is quite common for elderly and sick to not throw away things regardless of their utility (or lack thereof). Some people just can't stop acquiring stuff.
For collectable items utility does not matter at all. In many notorious examples rich people had many more microwaves cars than they could ever use. Examples include Bashar al-Assad -- former president of Syria with his reported collection of 100+ cars, as well as Viktor Yanukovych -- former president of Ukraine who reportedly grew his personal wealth to 30 billion(!) during presidency and who also had a large garage with many luxorious cars well beyond his needs. Greed knows no boundaries.
Furthermore, money don't have Marginal Utility. Those obsessed with growing wealth can continue doing so without any constraints, even ethical, to the limits of their abilities, even if they can not physically spend what they already have.
So while "each person knows his or her own wants and needs best" is true to some extent, it is certainly not universally true (e.g. for children, elderly, mentally sick, very rich, etc.) Boundless greed is very common when people have access to resources beyond their needs, as evident from most corruption scandals.