Comparisons like that always assume the that price is constant with increased demand even when world GDP is spent on something. What would likely happen is first the price would go up and later down as the world manufacturing would shift to vacuum tubes production.
Yes if a government started buying trillions of vacuum tubes, companies would quickly get much more efficient at producing then and there would be a huge investment into researching alternatives. But that brings up the question, what tech could we be subsidizing today so we can get it faster?
We already subsidize renewables, and that is encouraging research into them. The current projection of price per watt for solar is set to plummet in the next decade and a half. It's possible that further subsidies would encourage higher levels of production and economies of scale would reduce the price quicker.