These are two different things: {{DISPLAYTITLE}} defines a formal name for the header over 'From Wikipedia, the free encyclopedia', while {{PAGENAME}} and alikes return the technical piece of information - title under which the page is stored in database. For example, if $wgRestrictDisplayTitle is set to false, {{DISPLAYTITLE}} is not restricted to titles that normalise to page name, thus your proposal will make {{PAGENAME}} return a wrong title in links, etc for such cases. Recommend WONTFIX.

H92.wikipedia wrote:

OK, I’ll wontfix this. Thanks for the explanation. :)