That SA simply failed to source and store the spares is the problem here.
I agree. The government has admitted they are hard to get.
Using common components doesn't need to imply using someone else's spares though, the benefit is in the economy of scale and ensuring the reliability of future supply.
I don't know that there would be economy of scale here, this is specialised low volume stuff, there's no real scale to speak of. It's not like buy one, get one free burgers at Hungry Jacks.
I created some device that is used by some organisation here in SA, I was commissioned especially for the project, I quoted, then designed and built the requisite 10 solutions (plus one - which was not a spare*). This thing was mentioned to Victorian counterparts of the SA people and so the victorians wanted some for themselves. They asked me to produce 15 for them, a large increase on the original scale of the project. The Victorian order was quoted at 150% of the original quote and they got their 150% order. There are still no spares... The secondary customers did not save on the back of the first order, and the original customers were certainly not rebated for providing me with new orders. The only saving in the whole thing was to me and my R&D time because I already had the solution mapped, but that's not something that gets passed along, that's what pays my bills. If someone in NSW or somewhere internationally or whatever wants some, they'll pay the same way.
*Where possible, physically and contractually, I have always kept one of everything I have ever made, I don't do this for spares, I do it because I like to keep a catalogue of my work.