There’s the spice. Aiaiai 
Well… to your theory that if Meph were to be killed off the others might not appear at all, I say bollocks!
The others could be introduced in many ways, even as a coda to Meph’s “death”. Meph’s death could in some way trigger events that lead to Diablo’s awakening. Meph could be involved directly, or not even aware of the chain of reactions. Some other NPC could be involved. We could discover (in the following expac) that the vessel we thought had been Meph was actually yet another avatar thereof, and the real Meph was actually just siphoning essential life force from the powerful beings he confronted, adding their power to the various soulstones that will awaken all 3 brothers in a future confrontation meant to destroy us once and for all… meaning, we all played a role in reawakening them, and the Neyrelle thread was a weird distraction within the larger demonic ploy.
I’m typing these outlines while cleaning the kitchen & sipping tea. That’s how easy it is to come up with crazy & compelling threads that could be spun into stories. These writers actually get paid for cliche snooze-dramas with no resolution. 