Small...with no items that you don´t wear for chunks of time.
Matching...to facilitate this smallness.
Full of good quality items...so they last a long time.
And made of sustainable materials that will biodegrade naturally.
This is all well and good. But if some items (or most of the clothes you have) in your wardrobe starting off don't fit into that category you can't jump straight to the perfect wardrobe. Replacing everything would mean getting rid of everything...which is the definition of unsustainable.
When I did my initial clear out (and subsequent ones as I started by packing away all my summer stuff and office stuff and just doing a cull on my winter items-which I felt with later) as well as when I look over my wardrobe and pick out one or 2 things every so often my philosophy is to compare the value to me with the value to other people and if it is more valuable to me than it's reuse value then it stays to be worn out.
This has resulted in me having 2 pairs of tall boots for example (not essentials in a capsule wardrobe) because I love them, but they are not brand new, so if I tried selling them on adverts or sent them to a charity shop they might not get picked up by anyone. So they stay for now, but won't be replaced when they wear out eventually.
I also have (and then have gradually worn out) some t-shirts, cardigans etc, that I don't so much love as I know they have absolutely no value to anyone else (as they were cheap, starting to get small holes in them etc)...so I restricted my purchase of replacements and wore them (am still wearing some) until they can only be used as rags.
So my point really with this post is that it isn't sustainable to go out and buy a sustainable wardrobe.