So are companies like ARC, Conrad-Johnson, and others doomed for extinction as production eventually shuts down and existing stocks are depleted? Is the market for tube equipment because of lack of tubes going to disappear?
No, but the designs of old are being replaced by new technology that is not so dependant on old popular tube designs.
The Aesthetix Io used 4 12AX7 in the first gain stage and the new Audio Research uses a transistor. Other companies use a transformer or both transformer and solid state together. Audio Research went with four 6H30 dual triodes in place of 6922 or 12AX7. Older model Audio Research phono stages and line stages used 6922 but ARC abandoned them because of quality control problems from modern tube suppliers.
I'm surprised Aesthetix has not developed a new input circuit for the Io, it's a world class piece and among the best ever made but last I heard there were maybe one or two tubes in 100 that made the cut after sorting and testing for that first position.
NOS Telefunkens (back when they were available) were frequently good enough out of the box to do the job. Today even the "select" NOS Telefunkens have been picked over, tried and returned to the box until finding a truly NOS perfect pair or quad for phono is a massive undertaking.
The last time or two I needed them for my Io (before I sold it) Andy at Vintage Tube took months to get perfect stocks and he is among the best in the business at finding them.
I think there will be changes in circuit design to accommodate what's available. Those that don't change will find it increasingly difficult to tube up properly when needed.
All that being said, the old designs are magic, the new tubes like the 6H30 were designed for use by Russian Military and although they may test great and have fewer problems than new 6922 from Russia, they are far more electronic and solid state sounding.
Some newcomers are going with transformer coupled circuits combined with NOS Euro communication tubes like Siemens and Mullard to get reliability, low noise and great sound.
There are lots of ways to get there. Tubes are here to stay, at least for as long as most of us will be around.