Extraction shooters "needs" PvE-only mode as much as Dark Souls "needs" an easy-mode.
It's not a case of if they should offer exclusive modes. But you should get what you paid for. And you're paying for PvPvE extraction shooter. If they add a PvE-only mode down the line, fantastic, but their main focus, should be to create the best PvPvE experience they can in accordance to their vision of what they want the game to be.
No matter how much you playtest your game, there's gonna be "unexpected features", balance has to be tweaked, exploits have to be removed. They're gonna have their hands full for at least 1 year after launch before they can even think about adding additional modes.
Don't be ridiculous. It's never lupus.