Sorry but you are simply wrong. Marincin is a vastly better skater then him. It is his transition sksting that has caused him to be a liability playing against the rush that has held him back and will continue to hold him back. It is something that defines a D man as they are simply a pylon if their transition skating is similar to his. Finn had a high IQ but similarly bad feet.
Last thing you can not teach away bad feet it is an atribute that you are born with. If you could teach away bad feet it would happen with all similar afflicted players and it simply does not happen as it is a genetic issue.
Matt Finn had major injuries in his knees. He suffered an MCL and ACL injury to his knee before we drafted him, and then he had an injury in his knee before his first pro season that really sapped him of any sort of skating ability. Or at the very least prevented his ability to develop any sort of skating ability. Let's not compare Nielsen's skating to someone who suffered major injuries to his knees before he even got going. Those poor skating skills did not stop Finn from being a highly ranked defenseman either.
If Andrew Nielsen had such genetic and unfixable skating issues that prevent him from becoming anything more than a pylon, why was he drafted at all, never mind the 3rd round? I would think anyone with any sort of hockey knowledge would know when a player will never be able to become a good enough skater.
Finally, many of those poor rush moments stem completely from making poor reads and taking poor routes. Sure his skating prevents him from making up for it, but if he was smart enough to not put himself in those spots to begin with, he would not have such a problem against the rush. That's where you see the difference between him and Marincin. You see Nielsen give the puck away, or try to force things, or go out of his way to make a big hit and totally takes himself out of position. Marincin does not do that. If you have Gardiner level skating ability you may be able to fix your mistake through pure speed, but as we have seen with Gardiner many times, it still gets you into a ton of trouble.