The problem with guaranteed order is that you have to have some agreed upon counter/clock for ordering, otherwise a slow write from one producer to S3 could result in consumers already passing that offset before it was written, thus the write is effectively lost unless the consumers wind-back.
Having partitions means we can assign a dedicated writer for that partition that guarantees that writes are in order. With s3-direct writing, you lose that benefit, even with a timestamp/offset oracle. You'd need some metadata system that can do serializable isolation to guarantee that segments are written (visible to consumers) in the order of their timestamps. Doing that transactional system directly against S3 would be super slow (and you still need either bounded-error clocks, or a timestamp oracle service).