but facts don't really support your logic. OEL at #6 was the second d-man selected in 2009 (Hedman wen #2 overall behind Tavares), and Rielly at #5 was the third d-men selected in 2012 (Ryan Murray went #2 and Reinhart went #4). So neither OEL nor Rielly were the top d-men selected in their draft years.
also, 2012 was quite a year for drafting d-men. Yes, Takupov went #1, but thereafter there was a flood of d-men selected in the top ten, and the higher a guy was picked didn't necessarily equate with getting the better player. #2 R. Murray, #4 Reinhart, #5 Rielly, #6 Lindholm, #7 Dumba, #8 Pouliot, #9 Trouba, #10 Koekkoek. As of today the best 4 are Murray, Rielly, Lindholm, and Trouba. So if the Oilers pick a "Reinhart" at #4, that's not a win. The 2012 draft was quite an anomaly as it was a very weak year for forwards, and a relatively strong cast of choices in terms of d-men.
but you are correct in your assertion that in most draft years the top d-men (or two or three) end up becoming pretty good players.
for instance, 2015- #5 Hanifin, #7 Provorov, #8 Werenski; 2014- N/A; 2013- #4 Seth Jones, #7 Nurse, #8 Ristolainen; 2012- #2 R. Murray, #4 Reinhart- a miss. #5 Rielly, #6 Lindholm, #7 Dumba, #8 Pouliot, #9 Trouba; 2011- #4 Larsson, #9 Hamilton, #10 Brodin; 2010- #3 Gudbranson, #10 McIlrath- a miss, #12 Fowler, #13 Gormley- a miss; 2009- #2 Hedman, #6 Ekman-Larsson, #9 Cowen- a miss; 2008- #2 Doughty, #3 Bogosian- a miss, #4 Pietrangelo, #5 Schenn- a miss; 2007- #4 Hickey- a miss, #5 Alzner; 2006- N/A; 2005- N/A; 2004- N/A; 2003 (the golden year- #7 Suter, #9 Phaneuf, #8 Coburn- a miss, #14 Seabrook
but, the list also shows some misses, and the level of play of the top 3 d-men in this draft class (Sergachev, Juolevi, and Chychrun), I would suggest is a touch below that achieved in most other draft years (such as 2015, 2014, 2012, 2011, 2009, 2008), and say sort of on par with 2010 or 2007. This year does not have any high-end level prospect to anchor the d-men class, no Hanifin, no Jones, no Murray, no Larsson, no Gudbranson (although he is a miss), no Hedman, no Doughty.