I can’t see them being able to scrap any modern DMUs as they will still be needed, especially once 150s, 156s and 158s have to be withdrawn as life expired, which isn’t that far off, until widespread electrification in all parts of England (with Wales having its own fleet and Scotland being responsible for its own electrification) More pure DMUs are off the table, not least because they’ve run out of numbers for them.
I can see 195s, 196s and 197s staying where they are until they are withdrawn or displaced by electrification and then either be sold abroad or retractioned as EMUs. 153s will go first, followed by the remaining 155s and then the mass withdrawals of 150s, 156s and finally 158s will. There will be surplus 175s and 185s floating around to cover some former Sprinter routes, helped by innovative use of existing EMUs in the forms of 769s, 600s etc as a stop gap and perhaps battery equipped units such as 777s can absorb unelectrified lines which neighbour their area, but there will HAVE to be a lot of new electrification in the next 10 years if we are to stand a chance of replacing such a huge fleet of DMUs from the 1980s, with 832 carriages totalling 18,406.56m (excluding the 158s and 159s operated by SWR and 52209 and 52212 having been destroyed). Technically these could all be coupled together in a single consist.
The North East operations seem like a more natural home for 185s than 175s, since this is where the 185s predominantly run. The 175s are already cleared for and have already run on much of the network in the North West under FNW. Although they currently run on some Northern routes on long distance services from Wales, these are all electrified. GWR has got the 165s and 166s. 180s will likely just do odd jobs wherever they are needed.
Some infrastructural changes will certainly be needed though, whether this is electrification, or just because the only stock which will fit is life expired, either due to platform length or loading gauge. 4 cars is completely inadequate for St Ives. A 4 car 150 at St Ives was the most overcrowded train I’ve ever been on. In all likelihood though, with the Chester 175 depot going over to 197s, the 175 fleet, as well as the 185 fleet once TPE has finished with it, could end up being split up across the country. I couldn’t tell you where Turbostars will fit into all this. I’ve never even seen one.