Dont think this has been brought up but a couple of weeks ago the government published the economic assessment of removing the Golborne link:
The below figures are solely for the Golborne stretch if it was a wholly separate scheme including services to Scotland operating on it that would not otherwise exist due to lack of capacity, they did some light modelling of using the one spare Manchester path created by axing Scotland HS2 services for a London-Manchester via Birmingham Curzon service (the iTSS option), they think this would generate £100m net revenue and have £600-800m of economic benefits at zero change in capital cost for some reason.
Net Transport benefits: £4.7bn
Including WEI: £6.5bn
Capital Cost (including more trains and depot being large enough to stable Scotland trains as well as signaling improvements on WCML and a turnback siding at Preston): £4bn
Operating cost: £3.7bn
Renewal cost: £0.4bn
Operating Revenue: £3.4bn
BCR 1.0 and including Wider Economic Impacts 1.4
Phase 2B alone without Golborne being built BCR 0.5 including WEI 0.7
Phase 2B with Golborne included BCR 0.7 including WEI 0.9
Phase 1+2A BCR 1.0 including WEI 1.3
Phase 1+2A+2B excluding Golborne BCR 0.9 including WEI 1.1
Phase 1+2A+2B+Golborne (my own calculation using the figures provided as they dont calculate this figure)
Transport Benefits £40.2bn, WEI £53.7bn, Total Costs £67.4bn, Net Cost to Government £45.7bn, BCR 0.88, including WEI 1.18
This ultimately means the Golborne stretch improves the benefits/cost ratio of the whole HS2 scheme by 0.08 (7%) and of the 3B stretch in isolation by 0.2 (40%) & WEI 0.2 (28%)!
Essentially they would be stupid not to build it unless they can create a scheme that costs the same or less and produces more benefit, if none of the alternatives they come up with are better they will have to build Golborne unless they are economically insane.