New Train Testing Time

In an article I'm writing for Underground News, I note some facts on train testing times. In 1938, it took six weeks from delivery of the first unit of new stock to entry into passenger service.  For the Victoria Line's 1967 Tube Stock, the same process took 5 months and, for its new 2009 Tube Stock, it has taken 20 months.  At the same rates, for the next tube stock for the Victoria Line, due in the year 2050, it will take 6½ years to get the testing done. This is longer than it took to build and deliver into service the whole fleet of 1967 Tube Stock. Are we so unsure of our engineering that we have to waste so much time checking it?