Every time somebody makes prospect evaluations, things change. I remember some times when the Blues had highly touted prospect pools that ended up terrible. It's incredibly rare that these pools full of apparent can't-miss guys even mostly hit. And, miraculously, the Blues keep producing impact talent. Not only with higher round picks, but Parayko, Blais, Binnington, McEachern, and Sanford all came out of nowhere if you go by lists like that. Mikkola looks like a potential gem, too.
I think what happened with Kostin is that he was mentioned over and over in the same breath as Thomas (because he was picked just eleven spots later in the same year). As a result, people examined him against very high end prospects and came away thinking he's worse than he really is. Prospect journalism has never been about projects or depth guys who play with grit, it's all about attaching your name to the prospects people are most excited about. The highest scoring and/or the prospects of the popular teams. Most people don't read those to find real insights or to hear they might be getting a really good 3rd liner in a few years.