While I'm generally in favour of the principle that rail passengers, having chosen to travel by train, should be able to stay on a train for their full journey where possible... are we seriously suggesting that that should extend to dragging a dead Pendolino through Blackburn and Clitheroe, then over the S&C?
That's the only way I can see that you'd do that diversion, and given the lines in question and (presumably) relatively low speeds throughout - it's bonkers.
IIRC, passengers will generally take a half-hour hit on journey times to avoid a change. Going from train, to RRB, back to train is in effect two changes - so if the diversionary route takes more than an hour longer, it's a loser. Even if there's a big shed of suitable trains sitting around waiting to be needed, which there isn't. I think that's probably a sensible place to draw the line between diversions and replacement bus services.