Unless, god forbid, the sleeper services are withdrawn, there will be a need of new mark5 LHCS stcok eventually.
It is likely that the sleeper services will either be withdrawn completely at the end of life of the existing Mark 3 rolling stock, or they will be replaced with electric multiple units. By then it is likely that Aberdeen will have joined Edinburgh and Glasgow on the list of the all-electric destinations and the remaining trains will either be eliminated or hauled from end of the wires to the destination by attached diesels.
The former is made far more likely if HS2 does reach Scotland, because it cuts two hours or more off the travel time to the Scottish destinations (a late night crack express could probably do it even faster).
The sleepers alone probably do not require enough to make decent ecconomies of scale for an order of LHCS stock, and neither do the daytime routes where LHCS is the best option.
I'm afraid that are not really any routes where LHCS is operationally the "best option", the realities of modern rolling stock with its low maintenance solid state traction packages and such is that the capital cost has come to dominate the cost of operations. Additionally it is now easy to fill seats by flooding the market with yield managed tickets at very low prices (thanks to the electronic economy).
This means that there is every incentive to run the trains as intensively as possible which means the flexibility to change formation lengths according to demand is almost worthless (as every vehicle will be working every service possible). I already posted a calculation along these lines in the HS2 discussion thread about why fixed formation 400m trains are far superior to 200m ones if we accept pairing in the peaks will occur a significant fraction of the time.
However, taken together, I reckon the sleepers and the daytime routes where LHCS is more suitable than units (long distance INTERCITY routes with limited stopping patterns, I'm thinking XC's north-east to south-west corridor plus London - Scotland/Swansea/Devon & Cornwall/Carmarthen/Pembroke Dock) would be enough.
The XC corridor is currently served adequately (from the operators point of view) by the Voyager units that are in play now. Any additional rollnig stock requirements, such as to replace the HSTs, will likely be sourced from those ICWC Voyagers displaced by additional pendolinos and possible extensions to the electrification on the west coast.
Failing that there are the 222s at EMT which will be displaced at some point relatively soon by MML electrification, (likely to be replaced by more units).
In the long term there is the prospect of complete electrification of the core Manchester-South Coast route which would allow dual voltage multiple units to relieve those Voyagers which can then be consolidated onto the remaining NE-SW route, especially if eVoyager goes ahead.
DRS apparently has an option for more of the new locos they have ordered, it doesn't look like they think demand for locos will reduce very much.
DRS is after the market the Class 67 should hold if it was not a dead end, thanks to its obscene axle loading.
As I said the majority of the Class 67's non-freight duties are ones that are going to become obsolete fairly soon once the Mark 3 fleet starts to dwindle.
Remember Freight Operations are not really under government purview, this is simply DRS trying to make a play for market share at the expense of DB/EWS.