It should not be one or the other
To try and bespoke build all digital systems via GDS is absurd - just as buying them all would be.
Each case should be looked at individually, is a bespoke system necessary? Or is there an off the shelf product that can do the job for less cost?
Open source should be a necessity, regardless of the system, so that the government doesn't get locked in to any supplier contracts and to allow for rapid changes to be made should they be needed.
The complaints made by Copeland in this article scream "we have to outsource everything!" to me though.