I remember final content from the current expansion was a challenge for 5% of the player base, with maybe 1% actually seeing the final boss during that expansion.

Probably 25% of the player base was one or two raids behind whatever the final raid zone was.

And 75% of those who were actively raiding, were still in content from the previous expansion. Not geared enough to do the starting raid, for the current expansion.
But then, you would usually have around 1/3rd of your gear from the current, previous and expansion before the previous one.

It was not like Warcraft, where an expansion is a complete gear reset; to put it into Warcraft terms, your BiS, for six of your twenty slots of gear might be from Burning Crusade, and eight of your twenty slots BiS from WotLK, with the final 6 BiS pieces from Cataclysm bosses.
And the final guy in the hardest BC raid, would still be a challenge for 75% of the server.