Well the 1986 stock didn't exactly lead to a great production train (1992 stock).
Trains like 86TS add many years and much cost to the overall renewal programme. Better in my view to concentrate on a proper process towards the train you really intend for fleet builds. Things like 86TS never get fully sorted because everyone knows they are not going to be just genuine pre-production.
The 86 stock was more about trialling new concepts I think rather than producing a final product.
The 09 stock didn't have prototypes, just pre production units, and they ended up on a field in Cumbria! It's probably one of those things where it is hard to win - unless procuring a tried-and-tested design.
Most of what passengers care about is what they see and experience above the sole bar, which can be previewed, tested and refined using static mockups much cheaper than procuring one-off prototype units.
The essential things in life are seen not with the eyes, but with the heart. --Antoine de St. Exupery