I can answer! It is pretty difficult, yes. You need a lot of special equipment and near clean-room conditions. However, it is not insurmountable. The main reason (IMHO) that Russia became the defacto source for tubes is simply because they still make them for all their old gear and nobody else presently does.
This is not just for hipster guitar amps. The market for tubes includes hipster amplifiers, Nixie tubes, oscilloscope tubes, old radio enthusiast tubes, vintage HAM tubes, VF display tubes, dekatrons, and so on. It’s not a huge market, but probably enough to sustain a small manufacturer or two. None have popped up because none were needed (I think)- the Russians were supplying us what we needed. However much like boutique vinyl production ramped up quickly in the US once the market appeared, I suspect the same could happen for tubes. The demand is real, so there’s money to be made. There will be shortages for a while, but someone will take the opportunity.
