FWIW Python was fourteen years old when GVR joined the Borg. That doesn't address how popular it was but I think it's reasonable to say it was well established.
Guido van Rossum doesn't appreciate FP and he really genuinely doesn't understand it. That's not to say he's dumb or not a nice guy or anything. It's just not his area. And this is reflected in the language.
His attitude of you have to be "really smart" to understand FP is a mistake.
