I could be misremembering, and I'll have to dig a little more, but I thought I read that the TOS designs were to be very modular and that engines and bridges and other parts could be swapped out with relative ease?They'd use the simplest forms that get the job done and stick the part together most straightforward fashion possible
That's why I liked the Josephs' designs; they really felt like a Starfleet that was very modular and flexible.