As someone with perhaps more experience than many on this subject. Everquest had a crippling problem balancing Slow beyond it's first expansion. Unslowed bosses were effectively unhealable for more than a few seconds with more resilient bosses often warranting a literal meat pile of suicide tanks whilst it was slowed. Even higher level trash mobs could often be brutally dangerous without slow.
There's no denying that EQ went in with values that were far too high with the ability ending up like a bad drug addiction, but MMOs have steered well clear of that ever since. It's a shame as Everquest's debuff and CC game was actually one of the most interesting aspects of it's combat. But I highly doubt SE want to open that can of worms on EX/Savage content.
TLDR for this 2am ramble = Don't put any stock in getting a maintainable PvE slow.