Perhaps lack of sufficient income would be a factor too? I strongly suspect that many overhauls could progress rather more rapidly were there more money to wave at the job. But your basic point is completely valid. Reminds me of the tale from K J Cook (head of Swindon Works) immediately post grouping when he was criticised that there were 100 locomotives over conventional allowance awaiting overhauls, so perhaps they should be scrapped and new ones built. Cook countered that since all the services had been run they should scrap 100 and not build any new ones, and in the end that's roughly what happened.