Whatever the arguments for or against keeping sparse services running on rural line, the overall premise of the original question is faulty.
You won't save anywhere near 10% of costs by closing the least used 10% of the network. These lines have low maintenance and renewal costs per track-km compared to the busy lines in built up areas (as shown in
this ORR report). It's helpful to note that the Beeching cuts fell hardest on rural lines that had particularly high maintenance costs - the ones with massive structures such as Belah viaduct on the South Durham and Lancs line. So the remaining rural lines have relatively few big structures to maintain.
Likewise the cost of passenger train operation is low, with a small number of short trains which are long-depreciated and a minimal number of diagrams. All together I doubt you would save more than 1% of the total costs of operating the network. Total running costs were £26bn in 2024-25, £4bn of which were financing and other expenditure. So you're looking at perhaps saving a little over £200m per year. For comparison, spending on infrastructure enhancements and rolling stock in 2024-25 was around £10bn.
So we're looking at maybe a 2% uplift in enhancements from culling 10% of the network. That's basically a rounding error.