Maybe there are ways to reduce people losing interest, however. Just as an example (this may be impossible to implement, I have no idea) but if there were a level cap for the equipment you could use to fight specific bosses it would keep the challenge and risk of dying, while still being possible to progress and earn powerful items.
So to put this idea into perspective, let's use examples from Fenix 3.0:
The Ent: Dropped Tier 1 items upon death. So with the level cap I'm suggesting, you can only kill the Ent using items that are below Tier 1 (i.e. normal enchants/gear).
The Fire Elemental: Dropped Tier 2 items upon death. So with the level cap, you can only kill it using Tier 1 items or below (i.e. only items dropped by the Ent or normal gear etc.) - Basically, you cannot use gear dropped by the FE to keep killing the FE over and over again - the challenge has to remain as a result of only using the items dropped by the Ent.
Again, this may not be possible to implement, and the rewards may need to be edited to give at least some chance of beating the next Tier (or not overpowering the player), but a method like this would stop the grinding of specific Bosses, like the Ent in 3.0, and as a result people wouldn't lose interest.
And besides, if the interest remains for long enough, I would imagine that if the Mythic Bosses do get re-installed, people would be keen to see new ones also be added into the mix *Hint-hint*