I don't think it's completely ridiculous. It's clear the goal is to stop DMU use under a majority wire route, it's just that OP hasn't come to the right solution. I'm not going to pretend to have the right answer to the problem myself of course, but that doesn't stop me from making suggestions, and neither should it stop others, becuase I believe the cause is noble and I would hope we can find an answer together.
Agreed - there's a genuine problem here
Northern haven't had a lot of spare DMUs but currently diesel trains do eighty two miles under the wires to get from Manchester Airport to Oxenholme -
https://www.realtimetrains.co.uk/service/gb-nr:C22379/2022-01-16/detailed
So changing the services would have the double benefits of freeing up more DMUs (at a time when there's a surplus of EMUs, e.g. 350/2s coming off lease elsewhere) and being better for the environment by removing some diesels from electrified lines
But these services have two markets that need considering - the "left behind" towns potentially losing their only service to a big city is tough to justify politically - and also the Mancunians wanting leisure trips to Cumbria can be sold as an economic lifeline to those towns (so it's a bit different to, say, Kirkby/ Ellesmere Port losing their direct Manchester service, given that Kirkby/Ellesmere Port retain a regular Liverpool service and there's not many Mancunians going for a day out in Kirkby/Ellesmere Port). As Manchester has more and more investment, the gap between it and the rest of "the north" grows wider, you could argue that the need for direct Manchester services is more important to a town like Barrow than ever.
There's also the operational problems - do we want the risks of splitting/joining services at Lancaster, given the problems you get if trains miss the tight slots through Castlefield, how do we accommodate some infrequent services on a timetable packed with simple clock face services (e.g. the Manchester Recovery attempted to keep half hourly services from most corridors - other than Southport of course, but Southport passengers are too important to be constrained by such things)
Over time the service pattern has changed a bit, e.g. Windermere going from a bi-hourly175/185 to Manchester to losing most of it's Manchester trains when the "Scottish" services were given to TPE ... but then those Scottish services have gone from a handful of trains a day in the BR era (some of which were two coach 158s) to a bi-hourly service with Voyagers to an hourly service provided by brand new five coach EMUs... so the need for direct Windermere/Barrow services is perhaps diminished since a lot of the intermediate places have a far superior service from the 397s than they did with the irregular 158s
If Northern had some bi-mode trains suited to middle distance journeys then, great, allocate them. But 769s are seen as more like a 150 (in terms of lines they are allocated to), there's essentially zero chance of some "Flirts", so where do people come down between "through journeys are most important, the economic need of Barrow/ Windermere matter most" and "the railway needs to be greener and more reliable and therefore build timetables around simple timetables that see as many trains running on electric power as possible, we can't give every little place a key slot through Castlefield"?
I like a problem like this, there's a clear "problem" to solve (rather than people concocting some supposed "need" to spend hundreds of millions of pounds linking two minor rural places), it shows the kind of priorities people have (I'm not knocking anyone's priorities here, I mean that there's no "right" answer, we are all just trying to balance up the competing needs... )