Depends how you look at it. A 4 car 380 replacing 2 and 3 car 158s will increase the fleet size if the 158s are sent away. However, I can't imagine Scotrail getting rid of the 158s when they have 48x156s and Scotrail is actually in the position to choose which stock it gets and loses.
I think it would be tidier if Scotrail were to send their 158s south following completion of the Central Belt electrification rather than the 156s, as it would avoid "divvying up" the former Scotrail 156 fleet in such a way that some operators that could best use additional Sprinters (FGW, ATW, EMT, Northern) would gain an additional class of trains in their fleets, whereas all of those operators already operate 158s.
However, of course in the real world it is all speculation as to how many trains Scotrail will actually decide to release, and what form those trains will take. We'll have to wait a few years to find out.
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Yep - don't see many new trains in the next 10 years, and don't forget those diesels are the workhorses of the WCML - most of the trains I see on the WCML north of Crewe seem to Voyagers!!
Though that's partly because the frequency of West Coast services north of Crewe, and more accurately, Weaver Junction (At Crewe the ratio is 60:40 in favour of Pendolino operated services, and between Crewe and Weaver Junction, after the Chester & Holyhead services have "turned off" the WCML the ratio is two thirds to one third in favour of Pendolinos) is much less than that on the Southern section of the WCML. That the 221s are "the workhorses of the WCML" is far from an accurate interpretation when the full scope of the services in the West Coast franchise is considered: Only two West Coast trains an hour out of ten along the length of the WCML are diesel services.
And already with the Birmingham to Scotland services there is no need for this to be the case, as that route is already entirely electrified. However only with electrification from Crewe to Chester, and the passing over of the London to Chester services to Pendolino operation, do you really begin to gain sufficient "critical mass" to order a further tranche of Pendolinos to operate those two routes, assuming that funds could be made available for such an order of new trains.
If demand for rail travel is to continue to rise then additional capacity will be required, and electrifying Crewe to Chester would ensure greater capacity for London to Chester (in the form of a new build of Pendos) and for the Crosscountry network through the cascade of 221s to the XC operation, while 158s operating in pairs on the North Wales Coast in the place of the current Voyager service beyond Chester, assuming that additional 158s could be gleaned from Scotrail, would be no worse in terms of capacity than the 221s (Though you would lose the first class element of the service). At the very least a build of pantograph coaches for XCs' and Virgins' Voyagers would go some way to helping these aims.