It's interesting, we see the likes of Colas & DRS utilising assets as they require, making good use of older locomotives with relatively low values, and sourcing stock based on the work they have. At the other end of the scale is EWS/DBS/DBC, who to the casual observer have maintained consistent policies of massive waste of existing assets, over provision of replacement assets (66 & 67), and an attitude of such complacency that actively persuing work would seem to have been considered beneath them, giving away contracts to the compeition at quite a rate. It's no wonder they're in a mess of late.
I can't help thinking a lot of it has to do with the financials relating to EWS' original acquisition of these locos upon privatisation.
Class 60s were something like £1.2m each to build (late-80s prices). Class 92s were c.£3m each (ditto). EWS (IIRC) obtained a sizeable chunk of the fleets of these locos only a few years into their 40-year design lives. I don't know exactly how much for, but I believe
considerably less than the real cost of these assets to BR (aka the UK taxpayer).
From the EWS finance department's perspective, the asset value (and hence depreciation/cost of this) wasn't that great - so running costs etc. would be the main focus. Hence, the numbers would've stacked up more to just go and buy lots of new cheap locos from EMD instead. If they'd paid something more like a fair value for the traction they acquired, I can't help thinking they'd have invested more in using them and looked at ways of overcoming the challenges faced when the freight market changed.
On the other hand, companies that were in more of a "start-up" position (Colas, DRS, GBRf) had to acquire what they could and make use of those assets. All three companies have demonstrated that traction such as 60s, 92s and older kit such as 37s are still more than adequate if the right effort is put in.