Storage boxes, to me, are the biggest sign something's gone wrong. If you need boxes to store things you've got too many of them. And then you have extra boxes, and storage for your storage, and suddenly you need to put your card catalog somewhere. For me it's just, you've always gotta be pruning, you've always gotta know what's important to you. Don't own the things that aren't important.
Don't really agree on boxes - there's stuff that I do that's very important to me that needs a relatively large amount of kit that only comes out occasionally. Like various outdoor things - trad climbing, camping, winter hillwalking etc. An axe and crampons and bivi bag and four season sleeping bag and ropes and trad rack and bouldering mat and so on take up quite a bit of physical space but there is no fucking way I'm buying into any idea of decluttering that means getting rid of them, so I'm more interested in ways of having that stuff, not accumulating it unnecessarily (like not buying a new set of crampons every time a new model comes out that's meant to be marginally better in some way) and keeping it somewhere where I can get at it when I need it but where it won't get in the way of the stuff that I need on a daily basis. So decent storage helps.
I think knowing what's important to you is the interesting thing, though. It often seems to be presented as a relatively lightweight question - like, hey, you've got all that tat that you bought years ago and don't care about, if you get rid of it all you'll have loads more space - but in practice (for people I know well, at least) it seems like it involves much harder decisions, either admitting that some good intention that you have is never actually going to come to fruition - you're never going to mend this thing or use that thing that you could in theory use to make yourself a better person - or that you can cut out some thing from your life that you've sunk some amount of money into and actually quite enjoy and have probably integrated into your sense of self to some extent - like giving up a sport or a hobby musical instrument or something - and trust that the long term benefit of having more time and space for other things will make up for the loss.