Once electrification starts to filter through, building additional electric stock would possibly be more viable and extending electrification might also release more deisel stock. However, is anyone going to buld new deisel stock anytime soon?
Commentators such as Roger Ford of Modern Railways repeatedly suggest that no new
DMUs▸ are likely to be built for the foreseeable future.
There are many permutations of DMU cascades available following various electrification projects, the recent 'trans-pennine north route' ought to release a fair number of 185s (or 170s) for instance. Then there are a number of electrification plans within Scotland, which ought to release 158s or 170s. Also, Thameslink and Crossrail inject massive numbers of
EMUs▸ into the total mix, and many EMUs will become available from areas currently electrified for use elsewhere - this may require further electrification to soak up relatively modern EMUs that are additional to the well publicised 319s expected to come to the
GW▸ . At some stage infill third rail projects should become viable, in the case of SN this could release a reasonable fleet of Turbostars (171s).
I think what can be sensibily predicted is that all the existing 'better quality' DMUs with a reasonable life expectancy, such as Turbos and Turbostars, will migrate elsewhere, and indirectly release Pacers while maintaining the steady state numbers, the real difficulty is how you improve capacity as well...
Paul