Peter, I can see why you might want this. <sigh> 15 chars is the recommended "safe" value because anything longer *may* have deleterious effects on other elements on pages, lists, and other boxes - resulting in ugly wrapping of icons, images, etc.
This change carries some risk of messing up UI formatting but my limited testing didn't find anything obnoxious. It is also possible, in the future, that tight tolerances could be fixed with CSS or other means. That said, I do not expect to extend beyond the new max of 20 chars due to the ever-increasing chance of a wider-ranging negative user UI experience and expensive maintenance efforts around "CSS or other means".
I expect members can find a way to abbreviate their username within this limit.