Do wool quilts tend to break down and flatten out?
Do you mean a wool quilt, like a blanket/ duvet,
Or do you mean wool in the mattress case (wool quilted into mattress case).
In general, thicker loose wool will develop body impressions, which is more common for thicker wool toppers. That’s normal for wool, and doesn’t change it’s behavior so it’s not the same as foam softening. If the wool is precompressed, it’s less prone to do that though it changes the feel of it a bit… Quilting it into a top layer is a common way of precompressing it, but the degree to which it’s precompressed depends on the quilting.
I am referring to wool quilted in a mattress cover.
The wool that is used in the quilting of a mattress is generally thinner and pre-compressed to use as the fire barrier so it wouldn’t compress as much as a thicker wool topper. I think dn covered most of this as well as I could.
Wool is a very durable fiber so it certainly won’t break down easily or quickly … it will just compress to different degrees over time.