That's what the over 300 CG Reserve is for.
Here are some other things a +20 surplus helps with:
When someone needs a new computer for something, they consult a catalogue and get something tailored to their task; rather than looking up where the last available computer is running off a production line.
Replacement parts are available locally, not having to be tetris managed across the whole planet.
We are allowed to hand out Goods for private enterprise, which is now generating enough income to activate ~7 dice and growing.
Being at +0 just means that our production of capital goods somewhere inside the orbit of the moon is equal to our consumption.
That doesn't mean that everyone is getting the parts need in a reasonable time frame. (Although our large Logistics surplus would help there.)
Being at +20 means that hardware is abundant and that we can use the best parts for the job at hand, not just the bare minimum of whatever is available.
It means that supply chains can be optimised to not require raw materials to be shipped three times around the planet before they become deployed hardware.
And this is all rather pointless when we can generate another +12 Capital Goods next turn to cover the costs of PMM.
I never said don't do the PMM.
I just said that I don't think we exchange one good thing for another. (Especially not when we use a die to do so.)
Build the Capital Goods to cover PMM, and we lose nothing.