I am making the assumption that one who flies often in the system is
more proficient and experienced. Proficiency and knowledge, when
coupled with

You're confusing the concepts of "proficient", "experienced", and
"knowledge".

Lots of experienced, proficient pilots out there with no knowledge of
obstacle clearance requirements. Lots of experienced, proficient
pilots with lousy radio technique. Lots of experienced, proficient
pilots who don't understand how airplanes fly.

retention level is low...Is that so? Have a study to back this
up?

Six years instructing, and sampling knowledge levels after training is
over? But any learning theory book will supply you with the studies
you seek, if common sense doesn't.

IMO this would depend on the student and what they actually do
with their IFR rating once they receive it.

Not in this case. The only way you can reinforce your knowledge of
ODP's is to hit something every now and then. Until you do, this
knowledge is merely theoretical.

I don't disagree with the answers you received on this question, but
you bought into the idea that turning to the heading is "close enough"
without any idea of whether the posters knew what they were talking
about.

You can learn a lot from _IFR_ and _IFR Refresher_ but the knowledge
level of the authors is highly variable. I dumped "Refresher" after
some random CFI wrote a "Pitch vs. Power" article. When I want that
sort of analysis, I'll turn to aerodynamics texts. I stopped taking
"IFR" after I noticed that so many of their quizzes contained
incorrect answers. These guys are supposed to be experts?

Opinion from experienced pilots can be useful, but you need a way to
discern the good stuff from the bad stuff. Unless they rigorously
work to improve their own knowledge, they're as likely to be as full
of crap as the newbie -II, maybe more so.