Religions need some counterintuitive elements to elevate the system to a spiritual religion. If it completely made sense it becomes mundane. That is one reason the JW church fails to invoke strong feelings of the numinous. Its power to influence is centered only in repetition and group dynamics. That works ok but rarely does it evoke a sense of the 'divine' or deep inspiration.
The Trinity doctrine certainly appears to be an overlay upon the Bible, but at the same time it better explains a great number of passages than Arianism. It is for a reason it is regarded as the Mystery of the Trinity. It isn't supposed to be obvious nor easily grasped. In the world of religion that doesn't mean it isn't 'true'. The earliest Christians believed it was their place to decode the sayings they inherited, to be inspired by them, not to literalize them, to neuter them to becoming merely texts. This why Christianity bloomed into a rich diverse cultural movement. It took centuries for this to become muted through hierchal authoritarian leadership which sought conformity and dogma.
Yes, the doctrine of the Trinity in its final polished form was voted into canon many years after the writings that inspired it, but it is at the same time a product of the writings. Groups like the WT were born in a modern Western context and fail to understand the more ancient mysterious nature of early Christianity.