What an interesting post, Madmeg, as all of us men suffer from this condition - syndrome- perhaps, and it may seem a puzzle how you should deal with it. First let's think of some of the reasons. (1) One day soon I'll get around to using it. (2) Some day I'll get around to fixing it and then I'll use it. (3) It's too valuable to throw away (in my opinion) (4) It has sentimental value (5) It may come in useful some day. (5) I just like it.
There are many more reasons I'm sure but these are the main ones.
Your strategies must be flexible, sensible, hygienic, and cunning. First flexibility. You are never going to turn him into a minimalist - at least not until he's dead or comatose, in which event you will I'm sure say that you'd rather have him alive and messy than defunct and tidy. Make the resolution to change only the things which really need to be changed. Second, be sensible. The reason the best clothes are on the floor is because he wears them and secretly loves them. [Of course he should put them in the laundry]. Take him out and buy him some more nice clothes - which he must choose himself - and you will nudge out the old rags. Personally I have much-loved T shirts with paint and glue stains from my workshop, but I can see those staying with me for further use until herself forcefully encourages me to buy new ones.
Then hygiene. You as Directrice of Cleaning are entitled to have a healthy home and if you truly believe it's necessary, go in one day when he's out and clean it all up, vacuum the floor, dust, rinse off the artificial flowers, even tidy the cupboards but - and this is very, very important - however tempted throw nothing away. In fact replace it all in exactly the same place otherwise he will be grumbling for ages "Since you messed up my room I haven't been able to find my [Allen keys, spare staples, number 8 screws, blotting paper, empty Jo Malone boxes etc]." Next, use cunning. "Janet's son really needs an exercise bike - if you could clean off the rust he would love it." "Can we lend the electronic keyboard to our grand-son and maybe you could give him a lesson or two?" "Could we find some tools so the grand-daughter can take the broken printer to pieces - that's a lot of fun and that way you could teach her how to dis-assemble something." But don't do all that at once. Get some good storage boxes and put all the video tapes in neatly, labels up, so he can find them. Do you have a friend who really needs one of the coffee tables? (persuade a friend to take it, and ask him to take it round to her. Don't just confiscate it yourself) Papers to be sorted, but none thrown away. Magazines to be neatly arranged in piles. I do allow you to put the half-dead plants deep within the compost heap if you can replace them with identical but healthy plants in the same pots on the same day.
Accept that a man needs a space and clutter, but if you approach this in a clever and systematic way you can nudge him painlessly towards your idea of sanity.