If a new site been in the plan, any number of people would have tried to hold TfW to account for wasting money on it. Opposition MSs would have made great mileage out of yet another example of welsh government waste.
Believe me, as soon as accountants cost-engineer big plans, resilience is dropped very early to get the cost down. Accountants tell you liquidated damages will ensure the contract is met but generally aren't bothered with what happens if it goes very badly wrong 'cos that's for the lawyers to make money out of.
Few, especially not elected members, want to think in terms of likely to worst case outcomes when the optimistic case is much cheaper. Post-disaster 'lessons must be learned', but if the next similar big decision is 10+ years away by then you are back to square one.