This could easily be bypassed by players. We would have to limit searching for players to once per minute, or by name, or something ridiculous, and that's not user friendly. There will eventually be workarounds to even removing that - there are LUA interface events that trigger when a player loads on the screen. These could be used to automate the functionality between others that have a potential addon, and thus display actual numbers for other players.
Streams could be used to show numbers as well, ie; viewing someone's Twitch stream ingame can show the active zone.
Ultimately, it would be a war with addon / content creators. and one we would eventually lose.
From a gameplay perspective, knowing where AAO is becomes extremely beneficial to players who 'chase the AAO bonus' - you're fighting against the odds but are also getting rewarded for it.
Removing an indicator that you're outnumbered means that everyone will ask, "What's the active zone?" followed by, "Why is my experience/renown trash in the zone compared to <zone name>?" - both are not user friendly, and actually, there's a huge benefit to showing that - it makes the zone population get distributed evenly, and thus RvR action happens in some capacity.
There is a detriment to *not* showing it, as players have to ask each other, which would result in everyone going to the 'active zone' by word of mouth.
That being said, I do see a counterargument, which would be - 'players could be wrong about the active zone with players in it' - still, there will eventually be workarounds, and it's not worth spending development time on.