This is really the bottom line on Shatty. You can't name me 30 RHD that are better then he is, nor can you name me 60 dmen that are better. So, by definition he is top pairing.
He would be the leading scorer on some teams in the NHL this season (39 points in 57 games - 2nd on the Blues and out scoring guys like Duchene - who is forward.).
There is a very limited UFA class this summer - with only one difference making player - Shatty. And little to nothing available for D as a rental. Best rental player available by quite a bit.
He is the best Dman to reach UFA in a long time. Blues wouldn't even consider trading him if he wasn't a RHD - where they are stacked.
7 million is right on what most UFA top pairing guys are going to get paid. No one will be able to sign a top pairing guy for less then 7 going forward.... so...
Man, if you can add a player like him, your PP dramatically jumps and your overall depth at D will jump significantly also. Teams looking to get past the first round and make a run, you pay a 1st and top prospect and you don't think twice. He can make a difference on a contender.
Bubble team, yeah, I completely get it - may not be worth the add. But contender, you make a move for him.
Finally the chances of getting a top pairing guy are limited. So you will either develop internally (most prospects never hit their fan projected peak - so developing becomes very very difficult) or you make a trade that hurts bit. Getting him as a rental price, you don't bat an eye at because you will have a major advantage to other teams wanting his services this offseason and all things being equal - he will likely go with the locker room he knows and team mates he like and is comfortable in vs. bouncing to someplace unfamiliar.