Do you accept that the life expectancy of even the most modern of the DMU fleet will still be expired when there still will be unelectrified lines, some of considerable length, that will require something to provide their rail traction.
There will certainly still be some lines that will never be electrified, and will be in use long after all of the current DMUs (I think the most recent are the 185s) are life-expired. There will need to be some sort of long-term solution for long rural lines that get sub-hourly service frequencies and can never justify electrification. That, however, is hardly the point. In the short term, there are a heck of a lot of other lines that won't be electrified soon. A list of lines that will still be sparkless at the end of CP5, in the North West of England alone:
Cumbrian Coast Line, Carlisle-Barrow-Carnforth
Settle-Carlisle and Settle-Carnforth
Morecambe Branch Line, Lancaster-Heysham
Blackpool South - Kirkham and Wesham (being considered for tram conversion)
East Lancashire Line (Preston-Colne via Blackburn)
Preston-Ormskirk (connects to Merseyrail Northern Line)
Southport-Wigan
Wigan-Kirkby (connects to Merseyrail Northern Line)
Wigan-Manchester via Atherton (being considered for tram-train operation)
Manchester-Rochdale-Todmorden-Burnley (and the Calder Valley connection to Yorkshire)
Cheshire Lines Committee: Liverpool-Warrington-Manchester
Halton Curve and Warrington-Chester-Wrexham
Borderlands Line (Bidston-Wrexham)
North Wales Coast Line (Chester-Shotton-Llandudno-Holyhead)
Mid-Cheshire line (Chester-Stockport)
Chester-Crewe line (being considered for electrification)
Hope Valley Line (Manchester-Sheffield)
Buxton Line (Manchester-Buxton)
Helsby-Ellesmere Port (connects to Merseyrail Wirral Line)
Now, some of these should be done before any non-Pacers are life-expired: extending Merseyrail to Preston and Wigan, the mainline to Chester, the tram conversion of Blackpool South, the tram-train operation of the Atherton line. Whatever they end up doing on Borderlands to bring it into Merseyrail, CLC electrification, etc. All of these are in someone's list to get onto a future CP's programme.
But that's barely half of the lines in a region that is hardly massively rural - and that's at least 15 years in the future just to complete the projects I mentioned. You're going to have a similar story in Yorkshire and in the Midlands - and even less electrification in the SW or Wales. Only in Scotland, with STPR's massive programme, will non-electrics come down to just a few long rural lines (West Highland, Far North, Kyle of Lochalsh, etc).
There's going to be a heck of a lot of middling-use lines (1-2tph semi-rural lines and branch lines) like Southport-Wigan or the lines around Burnley that are not going to get electrified in CP6 or CP7 but which there will be a desire to get them wired up eventually in preference to buying expensive DMUs.
The rural lines can probably be operated by LHCS in a pinch - and there will certainly be freight locos for the long-term (or you can special order some DMUs with half of the end car as a power car; the demand on these lines isn't so high that you need to run a train the maximum length that the platforms will allow to deal with the passenger loadings). It's the semi-rurals where you're going to get very tight when the Sprinters start going out of service.
