I struggle to believe that anyone in the rail industry (let alone SRA, DfT, Treasury or whoever) would have suddenly decided: "Got a brilliant idea! Lets ditch nice, cheap Thunderbirds (that also happen to provide a more convenient service for passengers) and instead switch to expensive, inconvenient and 'uncertain' RRBs."
Do you have any idea of who could have forced such a perverse policy through?
The word you missed there was "unreliable"
If the Thunderbirds had coupled up to 390s reliably then they'd still be dragging them to Holyhead (and we'd have more Voyagers available for busy XC services rather than having to use them for WCML services from London to Crewe, best part of a couple of hundred miles under the wires
But even if the Thunderbirds could be trusted to haul Pendolini around, you're looking at around three hours from Carlisle to Preston via the S&C - e.g.
https://www.realtimetrains.co.uk/service/gb-nr:U63701/2022-02-26/detailed - noting that this train was pathed with a couple of dwells, but then there's no guarantee that a diverted service passenger would get first dibs at paths without at least some compromise/ getting stuck behind passenger services etc - plus you'd need to allow time for the 57 to couple to the 390 at one end and decouple at the other)
But, assuming that the Glasgow - Carlisle arrival and Preston departure for places further south match up and that you can couple/uncouple within the short period that the trains are dwelling at those cities, that's still a six hour duration for the round journeys between Carlisle and Preston - compared to around two hours for the direct journey via Lancaster
With two Avanti services per hour plus one TPE per hour, that's six trains normally in use between Carlisle and Preston, but you now need eighteen trains to maintain that level of service - i.e. twelve trains more
Plus presumably you'll want some trains to run any unaffected parts of the WCML too - e.g. if the reason that the line is closed is Shap then you'll still need to maintain some services from Carlisle to Penrith, or Oxenholme to Preston, so the fact that these bits of line will still need a service means that the dozen
additional units needed to maintain the service via the S&C will be getting closer to twenty - let's say sixteen?
Where are you going to take sixteen additional trains from, with similar seating capacity? Remembering that the Avanti trains will have to be able to keep to the existing paths elsewhere on the WCML, i.e. capable of running at 125mph which means being able to tilt
You could remove the Chester/ Holyhead services, but then does that mean that you are putting *those* passengers on buses instead?
And you'd need (say) sixteen class 57s, with drivers trained on the route? And more conductors for the Avanti/TPE services, given that more are required to maintain the same frequency
(if the answer is that "we don't need to run *all* of these services because I'm happy for other people to use replacement buses" then fair enough, but it feels like a lot of people on here are more focussed on getting to ride 125mph trains on rare track and scenic routes, without having to pay Charter rates)