The projected opening time for the high speed line to Birmingham is 2026. In the meantime about £25 billion will have been spent without a single passenger travelling on any of the route. The interest charges on this sunk investment are horrendous.
Compare this to, say, the alternative of upgrading and electrifying the Chiltern line to Birmingham, where the pay-back would begin as soon as the wires had reached High Wycombe, with further gains as the electrification included Bicester and Oxford, and more again when it got to Banbury. Add in a possible electrification from Basingstoke to Oxford and Banbury, and the approved electrification of the Great Western route from Paddington to Oxford, and the gains add up to a significant enhancement of the national network of electrified routes. And all of these generate returns on the investment as each stage of the project is completed.