Actually defence have not been the sole user of DMS assets in the past. The Seahorse vessels also did some commercial work and were available for other ad hoc government work as needed. DMS manning is, and will be, spread across its fleet allow crews to be rotated to vessesl with high work loads. Added to that, beyond merchant training the crew on these vessels do not need the specialist training needed to serve on a warship ..... which they tend to provide to Naval ratings and officers. Not an insignificant cost.
Yep the cost of the ship was considered in the contract, as it the cost of a building when you rent it. However, as a commercial asset the value can be offset by depreciation ....... not and option for a warship. This factor is also significant. By the way when you scrap a ship the return on about 1000 light laod tonnes is not going to be much.
Finally if the ship breaks ...... it is not Navy's problem, DMS (Serco) are responsible for this and bear the cost (and must provide and alternative for the services) with no change to the contract rate. Essentailly this process controls your costs and eases budgeting
Irrespective of who you may disagree with this is a cost effective solution (noting it covers a range of vessels) that has delivered new tonnage for submarine rescue, bunkering, training, aviation training and will provide an ice breaker .... in record time ....all without the bloated process surrounding warship builds that must pass defence approval processes....... which adds significant cost and is subject to a lot of politicing.