No reason why contractorisation will not work. Scope drift is indeed the prime cause of procurement pain and always has been. Just say No!
The predecessor of DE&S employed 32,000 staff when TSR.2 &tc. were chopped, early-1965. One group procured nothing but Army boots. Most of these bodies were established civil servants. Their Union negotiated, instead of a 0.5% scale uplift like last year, a change to the pension ("established" means pensionable) deal, to be inflation-proofed. Taken together with the then-unsackability of established public servants, the admin cost of anything-in-Govt. became intolerable: 25 years employment, 25 years pension. That's why contractorisation was introduced widely. When (for example) Bechtel completes a Task they will fire those members of the team who had survived Annual Appraisals, who will then port their personal Pension Plans to their next berth.