Actually elementals cannot die. Only their identities might disappear but if you kill them (Ragnaros etc.) their form will simply be destroyed and another one will reform later on with different name... you just simply cannot ever kill off elementals, but they do not reform like demons do in the twisting nether because elementals are natives to the real existence.
This was explained in the chronicles, every planet has elemental-forces (the more of it exist the more likely youll see them form into physical beings)... depending on how much element of spirit exists the elementals will remain either peaceful or chaotic, and since azeroth's world soul had absorbed most of the spirit they werent very peaceful, so when the old gods arrived the natives just kept waging endless warfare against eachothers, once one was defeated the others turned on eachother and the weakened one simply regained its strength and rejoined the fight later, it was literally neverending. (when titans arrived the old gods had enslaved the elementals)
The elemental plane as you said is made by keepers, but its not a realm where they can "die"... its a prison. They were imprisoned there so that azeroth could stay safer for the mortal races.
Obviously it takes strong magic and power to get out of the plane (sorcerer-thane thaurissan's ritual for example) and once defeated the elementals are banished back. Overtime they embraced this and created their own domains within.
I would guess if you actually kill ragnaros etc. then another will reform in the real world instead and potentially wreck havoc on mortals... thats why the big named elementals are imprisoned within their elemental domains so such power cannot easily exist in the mortal world.
Not entirely sure about this since ragnaros was killed(?), maybe they just retconned it or maybe we will face a new raggy later once his successor becomes strong enough.