can you elaborate on what you mean by 'does not look good'?

As far as I remember the dashes are reasonable at the default line width but do not scale well when the line width is changed. Is that the issue?

The dashes do not scale with the line width. I've often thought that was less than ideal. i.e. the dash "units" should be a factor of "linewidth", not in points as they are now.

Obviously a very backward incompatible change, but certainly worthwhile.

Yes, the dashes not scaling with the line width (or figure size) is the the problem. I'm not sure whether the dependence should be on line width or figure size. In my view it would already be an improvement to set the dash spacing globally which could be achieved in a backwards compatible way. Of course automatic scaling would be even nicer as it would be less configuration hassle.

Dash scaling was introduced by #5926.