Very much going off topic (sorry) but a lot of this stems back to how rail freight was viewed at privatisation. EWS (as was) had the near monopoly on rail freight apart from container traffic which was shared with Freightliner and then DRS being formed by BNFL to operate the nuclear traffic in house. They purchased a fleet of 66s to replace everything except the 60s (and 59/2s inherited from National Power). In hindsight I am sure a smaller fleet of 66s would have been purchased with some 56s or 58s being rebuilt had they known how little time some of the majority traffic lasted.
Back then coal was king, with significant oil, metal and other 'industrial' flows that have slowly dropped away. No one could have predicted the sudden end of mail and coal traffic which would leave DB with too large a fleet of modern locomotives, this has been compound since by the loss of other significant traffic such as the Mendip aggregate flows and Iron Ore to Scunthorpe.