I’ll second the pvp parts of this point.
Fun is the name of the game here. At the end of the day everyone plays the game to have fun, to enjoy themselves.
For the hardcore(and semi), a lot of what they determine to be “fun” is derived from feeling significantly challenged and overcoming challenges to win. Difficult content is what these groups enjoy. Not difficult in the sense of requiring cheese-strategy to win, but difficult in the sense of requiring skill, coordination and teamwork to win.
People miss worldbossing not because the mechanics of worldbossing were good or that killing the worldbosses was interesting/challenging, but because the competition against other players was very interesting. The organisation that went into it, the group coordination to secure the bosses, the work that went into tracking and spawning them, the extremely detailed and intimate knowledge of the game in order to compete with others that also wanted to secure those bosses. This was in itself a form of pvp, created by pve, a generated challenge that people enjoyed thoroughly despite optimisation issues.
PVP supplements pve endgame for these groups because when it is well balanced and cheese strategy/frustrating mechanics do not exist, everyone feels properly challenged. When people feel properly challenge they enjoy overcoming those challenges.
Endgame should adequately challenge the hardcore without being lazy with regards to generating artificial difficulty.
I think a very strong extra reason to bring back worldbossing is due to the social nature of it. Previously, worldbosses were an incredibly strong social part of the game. When waiting on worldboss spawns and competing with others you would naturally meet and group up with other players. Friendships were made due to worldbossing that I still have today even with some that have moved to other games. It was an excellent tool to generate social interaction between players.