The Hellfire Ring will still be valuable, though. It may not be as valuable to players who have reached Paragon 100 now that it doesn't deal buttloads of damage when it procs, but it will still be competitive. Given the nature of how the ring is crafted, it has the opportunity to roll with some pretty incredible stats, so one of the rings you create may be BiS for you. Then again, it may not, but that's a good reason to keep farming (if that's your thing).
As for players who are leveling Paragon, we're okay with Hellfire being BiS. Yes, it will help you level up faster, but even if you stack it with Leoric's Signet and a ruby gem in your helm, reaching 100 will still take a very long time. And, as you've illustrated, the ring probably won't be a "must-have" forever, at least not for everyone depending on the stats you roll.
I.E. what this guy said:
The Hellfire Ring can still be BiS, now you just need to rolls to do so. You already have High main stat....you just need the 4 randoms to roll trifecta. It will now take a LOT of farming to get BIS hellfire ring.
Yeah, but what are the odds of that? The chance of getting a trifecta roll in 4 random mods is (4 choose 3)/(32 choose 3) = 1/1240, and who is going to run that quest over a thousand times in the hope of a lucky roll? Most of the time you'll just get crap like increased pickup radius, extra health from potions, or bonuses to useless stats for your character, and people will finish the quest once, realize that they just spent a lot of time farming for an item that's worse than what they were already using, and never do it again.
As I said in a different thread, an epic quest like this should have a reward that is actually useful for your character, not a reward that has a small chance of being useful and a large chance of being garbage. IMO the ring should have two fewer random
properties, and two more useful
properties. That way there would still be an incentive to farm for a long time for those people who really want the perfect roll, but people who finish the quest once actually have a good chance of receiving a quest reward which is useful. Can you imagine how demoralizing it will be for someone who finally completes their hellfire ring after many hours farming, just to find that the ring shows red numbers in all three categories (life, damage, and protection)?