1) Software usability depends on cultural norms of the user.
2) Widely used software becomes part of the cultural norms, regardless of usability.
3) 1+2 means that the usability of a given piece of software can decrease (or increase!), despite the interface not changing in any way.
4) Without active cultural work, we can get "stuck" in a local maximum of software usability (i.e. better usability is possible, but not with the current cultural norms)