I do find it amazing that this seems to be the case in the rail industry - for almost every new build fleet. I'm in the shipping industry, where very complicated vehicles, often one of a kind, are routinely* delivered on time, to spec, on budget with crew already trained (crew training normally happens whilst the ship is still owned by the shipyard, and then on passage from the yard to the home port), and the ship enters service a few days after delivery. No doubt there is a good reason why this doesn't happen for trains, but as an outsider looking in it is difficult to see what it is.
*There have been a couple of high-profile exceptions recently - Caledonian MacBrayne's Glen Sannox is a good example of what happens if you let politics mix with shipbuilding (perhaps it helps explain the issues for trains, and recent deliveries from Flensburger shipyard, but since delays to Honfleur and WB Yeats led to the collapse of the yard, it perhaps just underlines the point.
What's the lead time on a new ship from design to delivery and entering service?
I made this point when somebody else mentioned aviation - but look at the Airbus A380 as an example - project announced in 1994, first prototype unveiled in 2005 (11 years later) with first (test) flight in 2005 - there was a 2 year delay due to wiring problems - it finally received its type approval from the EASA and FAA (European and US regulators) at the end of 2006 and the first delivered aircraft was in October 2007.
Compare that with the 230s - 2014 the D78s were bought by Vivarail, 2015 a prototype was produced and in 2016 mainline testing on the Coventry - Nuneaton line, ordered by WMT in October 2017 and entered service in April 2019.
Hmm, but the efficiency improvement with a new electric traction package is going to be nowhere near that from a 40 year old IC engine to a modern IC engine. Especially on a relatively low speed route like Island Line.
A more pragmatic solution might have been to introduce a couple of unrefurbished D stock trains whilst the "new" trains were being built and tested.
But may not work with the other infrastructure works being carried out.
Having an uncommon fleet would have added cost and complexity - and no guarantee it would have worked in any case.