Unfortunately "improving articles on subjects of note" (as well as arguing notability) requires more contentious editing/lawyering compared to "developing new articles on marginal topics". The lawyering is exciting for one kind of old-hand contributor -- but turns off many others.

So optimizing for "improving articles on subjects of note" may paradoxically serve to make total editor attention more scarce, and waste more of it on low-value disputes and incremental refinements. "Why bother," a newbie or loosely-attached casual contributor might ask, "when all these busybodies keep marking subjects of interest to me for deletion, and mangling prose with obsessive footnotes (or requests for same)?"

!#@!@^% deletionists are ruining Wikipedia. They'll be the first against the wall when the revolution comes.