I'm not sure exactly what you're asking here. It's not about 37s having a 'competitive advantage'.
Well, yes it is. With any other freight fleet, its clear that more modern locos are economically more viable - thus the removal of Class 47/56/57/60 (add BR loco of your choice) with Class 66/67/68/88.
Colas have a fleet of 37s, as do DRS (who I believe sold some locos off and leased them back). So if Colas and/or DRS bid for RHTT work and lost out because Freightliner or DB bid cheaper, and a loco's RA was not a concern then you would not see 37s on RHTT work any more.
But DRS keep them specifically for RHTT contracts, as opposed to sourcing something new. This is my point. 37s dont do any other regular work in any numbers as a matter of course.
Equally, Colas introduced a large number of 37s specifically for the test train contract.
If 37s are part of your fleet, why would you spend a lot of money replacing them with new locos (at more than a million pounds per loco, plus the associated cost of training drivers and maintenance staff on the new fleet)? How would that make commercial sense?
Because older assets generally cost more to run, more to maintain, harder to source spares etc. Why did DRS bother with 68s when they had perfectly good 47s? Because it made commercial sense. Why are 37s slightly different? The TOC equivalent would be to use a 101 today.
MPVs can only do 75mph, while 37s can do up to 90mph, while MPVs have more limited haulage capacity. You can't use MPVs on NR's test trains, where the extra speed a 37 offers over 66s and 70s is often vital, it's the same reason COLAS acquired 67s, while their better route availability offers greater flexibility. Barring a new build to replace the very much worn out 31s, nothing else would do the job the 37s can do as comprehensively as a 37.
Incorrect. Colas had a single MPV for test trains until very recently. 153s are being introduced for test trains. There are various other none 37 options. A worn out 31 is only worn our until its refurbished (just like half the 37s on the mainline today were 10 years ago). Why the 37s over the 31s is still a fair question.
With suggestions being that COLAS is on the lookout for additional MTU HST power cars (with some of the soon-to-be-stood-down GWR Castle sets being mooted as replacements) for test train work - and fewer now employed on autumnal RHTT - the long reign of the 37 on the mainline on anything other than heritage / tour work may well be coming nearer to a close than certain enthusiasts might like.
Can you provide a source for this? Why would the castle HSTs make a difference when there's no shortage of MTU powered Class 43s available? Would also seem odd as the ex-GWR 43s are wired differently to the ex-LNER ones, which inhibits some fleet flexibility options.
I'd have imagined if Colas wanted more HSTs, they would have sourced them from the many ex-LNER ones available and probably already done it - correct me if you know otherwise though.