It's a M+ nerf (and to a lesser extent, a PvP nerf with the Blizzard/Frost Orb snare change), but it's not going to ruin Frost Mages in any way.
Like we didn't even have a 15-second slow on everything before Legion except Frost Bolt. Frozen Orb was 30% for 2 seconds, Blizzard was 50% for 1.5 seconds and we had to channel it for the entire duration.
Melee snares in PvP are a separate issue and they are long overdue for nerfs.