Could they eventually replicate a CMOS technology? No one doubts this, but the latest lith process took how many years to develop, and only one company makes those machines anywhere in the world? Nearly microscopic molten tin droplets being flattened mid-air so that it can radiate a particular wavelength of UV?
That's not something they'll have up and running again in 6 months, and if it were lost, regression to other technologies would be difficult or impossible too. We might have to start from scratch, so to speak.
Vacuum tubes are still made. They’re used extensively in instrument amplification.
But I think this bolsters your point!
If there were some kind of interdiction on silicon (an evil genie or some kind of Butlerian Jihad perhaps?), the market would remember and/or rediscover the thermoelectric effect and throw money/postapocalyptic bartered goods at glassblowers pretty sharpish.
If that status continued, I'm sure we'd see developments in that space in terms of miniaturisation, robustness, efficiency, performance, etc., that would seem as improbable to us as a modern CPU would seem to someone in the no-silicon timeline. You may never get to "most of a teraflops in your pocket, runs all day on 4000mAh and costs three figures" but you could still do a meaningfully large amount of computation with valves.
