I'm not sure I completely agree with that for few reasons (provided I understand the comparison you are making):
1) Slavin has been a top pairing defensemen for a number of years on the NHL. Granted, he came to the NHL at a later age than Hanifin, but Slavin was playing as a #1 D / on top pairing at the age of 22 years old and performing extremely well at it. If he was in a bigger market, there would be way more hype.
2) Slavin doesn't ever need a "stable partner". He IS the stable partner. The reason Hamilton has so much success at Carolina is because he has Slavin as his partner so Dougie can take more chances and Slavin can cover up for him.
3) Hanifin has slightly better physical tools than Slavin, but Slavin's hockey sense, reads and quick reactions are what sets him apart from Hanifin. It was this way when Slavin stepped on the NHL ice at 21 years old and consistently been this way.
If you asked Canes fans today, if you could keep 1, do you keep Hamilton or Slavin, the answer would overwhelmingly be Slavin. Yeah, I get that he doesn't get PP time like Dougie, and isn't as offensive so many people call him a #2, but IMO he's more valuable and I'd have no issue if he was our #1.