While I was addressing arenas, you seem to address primarily open world pvp, so id like to address that specific point.
I agree that today less people think open world pvp is fun than back in the day. One reason was probably the tons of “open world pvp” videos and the overall more casual mindset of many players.
Back in the day servers became close to one faction servers, too, but not to a degree it is today. As said, in vanilla classic, open world pvp was used by so many people, that it became unenjoyable for many players, and as soon as one faction had significantly less active players, the faction side was bound to be doomed, since playing open world became borderline unplayable. This forced more and more players to swap servers, since the issue only grow bigger.
The biggest factor concerning mono faction servers in classic were the free and time limited transfers to any destination desired. This put pressure on the players, since they saw multiple big guilds leave, saw friends leave, and no individual wanted to be left behind on a dead faction side. This is a psychological effect causing a self-fulfilling prophecy that players cannot be blamed for, since it was Blizzards decision to make these free transfers available and without Blizzards transfer option, this would never have happened.
The reason servers became mono faction “faster” than they did naturally back in the day, is to a degree, as you say, the players mindset, but at least to a similar degree the server sizes and first and foremost the free transfers. Without the transfers, many servers like Firemaw, which was pretty balanced (around 40-60 faction balance), would not have never become mono faction servers or at least not within a few days, as they did. As far as I can tell, its definitely not 100% the players fault, and 95% isnt close to the truth either. With a faithfully recreated classic experience, with original server sizes and good server management with free transfers from full servers to small servers for specific factions to balance them out, I am 100% sure there wouldnt be only mono faction pvp servers today.