I adore the 'Leader'. It was the only genuine attempt to construct a truly modern steam locomotive that abandoned the Stephenson-pattern and sought to incorporate the best of diesel and electric practice with the well-known technology of steam, in particular, the modular bogies / power units that are such a strength of modern D&E traction could well have revolutionised steam locomotives.
Ultimately, I do not think that this would have 'saved' the steam engine or extended its working life. The steam engine was ended by a political decision and not a technical one - even if there had been a fleet of Leaders running I cannot see steam enduring beyond its historical date of 1968.
The designs that Bulleid suggested were fundamentally flawed in a number of respects, but we will never know if they could have been ironed out with 36002 which was stopped less than a week from its completion and incorporated almost all of the improvements from 36001. Ultimately, this would still have produced a locomotive which was phenomenally overweight and heavy on both coal and water.