99% is not 100%. So no, it's not the Forgotten Realms. A facsimile of the Realms, sure. But you asked if demihumans were necessary for the Forgotten Realms, not if they were necessary to create a human-centric facsimile of the Realms.I'm not so sure about this. If the game has the same pantheon, the same map and locations, heck, the same cultures (but the inhabitant of Gauntlegrym are Humans living in a super-fortress/forge, loving metal and beer instead of dwarves living in said super-fortress/forge, loving metal and beer), is it not 99% the FRs?
Edit: (Just to be clear, I do not have an answer to that. That's why I ask)
Pedantic nitpicking? Absolutely, guilty as charged! But hey, you asked.

Now that being said... if someone's idea of the Realms is the stories and events that have occurred, but not who did them... then sure you could change everything over to humans-only if it really mattered. Personally, I don't see the point of it myself... why go through all that effort to wash away all the demihuman races just to replace them with exact human replicas? Especially considering most people roleplay demihumans as humanistic anyway. But hey, maybe for someone that matters.