This was discussed on Monday’s stream. Treasure troops were singled out for removal to be a compensating adjustment for the final treasure troop drop rate being settled as higher that what was originally designed. As such, the treasure troop task was removed as a way to “claw back” some of the excess treasure troop drops over what was originally planned to flow into the game’s economy.
They are as optional as any other game mode in GoW. No one is forcing anyone to play delves.
On the other hand, if you are playing delves for the rewards, the main rewards are the faction pets that are gating kingdom Power. If you truly want delves to be optional, then you are at the mercy of waiting for the 2nd pet for a kingdom to be released naturally. A quick look at Tarans shows enough unreleased pets in the game files for at least two full more years plus cosmetic releases. In that case, I hope you have the patience of a saint because most, if not all, kingdoms are going to hit that hard wall at Power 15 over the course of the next year requiring that second pet.