The Jargon File (version 4.4.7, 29 Dec 2003):

creeping elegance
 n.

    Describes a tendency for parts of a design to become elegant past the
    point of diminishing return, something which often happens at the expense
    of the less interesting parts of the design, the schedule, and other things
    deemed important in the Real World. See also creeping featurism, 
    second-system effect, tense.


The Free On-line Dictionary of Computing (30 December 2018):

creeping elegance

   Describes a tendency for parts of a design to become elegant
   past the point of diminishing return, something which often
   happens at the expense of the less interesting parts of the
   design, the schedule, and other things deemed important in the
   Real World.  See also creeping featurism, second-system
   effect, tense.