Just as 'one person's trash is another person's treasure', each person will have their own ideas about decluttering. Some will believe they don't need to declutter at all, others will part with a few items and feel they don't need to do any more and yet others will cull until they have achieved a minimalist lifestyle. However, there are some who are so compulsive about decluttering they will even get rid of items they know they will need in the future.
Some people keep large quantities of particular items for a hobby or creative pursuit from which they earn money. They can see the potential in certain materials and will store them for later use. I can see the justification in this.
There are people who are researching a particular topic. They may accumulate many resources for this research and value these irreplaceable items.
There are numerous reasons some people make a conscious decision to hold onto items that another person would part with. Some people prefer to hold onto a greater number of items and it is because they have thought about it and decided against undertaking major decluttering. Decluttering or not doing it is a personal decision.
Once the decluttering process has been started, how does one know when one has done enough? Again, this is personal and not really something another person can decide for you.
Personally, I felt a sense of satisfaction when I got to the point where I could say I have decluttered enough. From time to time I still buy something for my house or garden but am more conscious of what I bring into my home than I was years ago. And I no longer keep something simply because I have had it for a long time. This is my approach to decluttering and keeping clutter from returning to my home but other people will have their own way of doing things.