People won't flag themselves PvP for many reasons, mainly because it's not very fair to be flagged as a walking target to others while they are not. It opens doors to many unfair situations for example someone can stalk you and call friends - the target can't do anything about it other than leave or run away from the zone. Also having yourself not flagged as a target makes you much more easier to start on the preferential side of the fight, for example hunter placing down traps, warrior making the first charge, etc. Telling people that they have an "option" to turn pvp on is like: "Yeah you can do what you enjoy, but with many side effects." Dumbing this down doesn't help the argument nor logic behind it, since you are talking only from one side of the argument and not looking into it from both sides. People will rarely flag themselves as /pvp on PvE realm, not because there are few that actually like wpvp, but because it's not a working solution.
There are no winners in this discussion, both options have several advantages and disadvantages. Most of it however will be heavily impacted by the high rates and RDF enabled which both did not exist on the original blizz realms. This will lead to a lot of ppl having alts quite fast and only remaining activity left will be to do professions or farm gold. Having both RDF/x5 at the same time will turn zones into farming spots - this could lead to some interesting scenarions where you can compete in PvP for resources, especially with so many people finishing 60 at the same time. Once again on PvE realm will be a lot of bots stealing your resouces and you can do nothing about it. With PvP realm you can at least kill / harass them, but at the same time from the pve player perspective you will be forced into something you don't like. Why I said this ? So people will finally realize there are always two sides of the coin and blindly siding with one side just because it matches your preference while shutting down ears to other people opinion is really "an interesting line of logic".
Also people should once and for all understand that this will never be a blizzlike vanilla server. It's rather heavilly modified wotlk locked into 60 level with QoL aspects that will drastically change the game. Anyone who paid attention should understand that by now, too bad not many people here do and still ask for x1 rates or turning off the RDF - none of these things will happen, I can guarantee that. If someone is looking for true vanilla experience, this is not the place to find it.