The eldar took it to the extreme and they had an empire that was the size of the eye of terror and they are major psykers. If they didn't exist slaanesh would of formed....just would be after 40k.
Actually their empire was galaxy-spanning, where the Eye of Terror now rests was just the 'heartland' of the empire.
As for Slaanesh being born of the Eldar, well yes obviously pleasure and excess was present prior to the decadence of Eldar civilisation - and Slaanesh has always existed within the warp to some degree. But it took the terrible extremes that the Eldar empire descended into to truly bring forth the Dark Prince.
Ardias- If Slannesh was never born, humanity on Terra would be stuck forever since the warp storms surrounding Terra would take ages to desist or stop.
The whole reason for said warp storms (A.k.a Age of Stife) in the first place was because Slaanesh was coalescing thus causing turmoil within the warp. Her birth ended the warp storms, their rage having been spent with her birth but ultimately she (via the Eldar's decadence) was the cause for them in the first place.
Without Slannesh = no galaxy spanning human empire, Eldar race would still be the most dominant xeno's nation and their empire would probably rule for millions of more years till some other disaster befalls them. ( 40k story line would cease to exist beyond that point and GW would be out of business. Lol)
Humanity was a galaxy-spanning race prior to Slaanesh bursting into divine consciousness, A.k.a Dark Age of Technology. And would generally probably be better off if it wasn't for the Age of Strife (which occured because Slaanesh was growing in power).
Did the beginning of psykers occur when the Emperor had many wives and mated and had kids which then had their own kids passing the psychic infused genes on to further generations?
No, humanity (like most if not all races fostered by the Old Ones) has always been a psychically attuned race.
As for lore I don't like or enjoy - well it's not exactly lore, but I really don't like the term 'Space Marines' it just sounds terrible. Hence why you'll always see me referring to them as Astartes; Legio or Adeptus. And I'm glad at least one BL author (ADB) agrees with me on this one and takes the same stance.