Re: Air gap
The problem with this idea is that almost everything created since the early nineties if not earlier does need to be networked.
You can try to separate “internal facing” from “public facing”, but you’ll quickly find that at least some of the important public facing services need to connect to the internal services, at which point you no longer have an air gap.
In any typical enterprise, even working out what the services are, which need to be internal and external and what connects to what takes time. In one very large and systemically important company I worked with briefly, they had been warned that their main data centre was unacceptably flammable to the extent it was uninsurable. A power incident had shown there wasn’t working DR for the most basic services. They had two lists of critical services - each around 1000, from simple infrastructure to e.g. multiple out of support SAP instances, barely understood but highly interconnected custom mainframe services, modern customer portals - they thought might be running in it. These lists overlapped by only about 40%, partly due to inconsistent system naming, yet were still incomplete.
It took 6 weeks of intensive effort for a four person team with wide remit to talk frankly to anyone in business, IT or suppliers to try to get to a point where we had even a basic understanding of what was running, who was using it, and how important it was to them. We didn’t even try to figure out the integration picture (person years of effort) and just used horribly crude assumptions to put a very approximate number (over £100m) for the board to kick off what turned out to be a five year remediation programme to procure three new datacentres, network and computer infrastructure, carry out proper analysis on and migrate or demise services without causing outages worthy of newspaper headlines. The last two were the hard bits.
This was a while back, but these problems never die as IT rots over time. Never underestimate how poorly legacy IT is understood, and how difficult it is to touch.