"guilds need to be able to complete objectives for in-game rewards"
This exists. Raid rewards, TB rewards, TW rewards.
Guilds have no purpose? Guild cooperation is more required than ever to score well in the raid, meet the star goals in TB and execute defense and offense in TW.
I disagree with OP's suggestion that TW rewards are somehow insignificant. TW (at a certain GP threshold) rewards droid brains (otherwise only available for monstrous crystal prices or through raid tokens and oh, you need a guild to get those too) and mk3 datacron rerolls (otherwise only available for crystals). The difference between winning (60 mk3 reroll materials) and losing (20) is also huge. Dcrons won't become less important. Every set introduces a few must-have, OP lvl9 abilities that players have to reroll to get using mk3 materials.
Character/ship shards are the most important rewards in the game, but most of the gear to build those characters is obtained through guild events. Darth Bane will do nothing for you at gear 8.