I'm glad he touched upon the matter of transistors being discontinued. Nowhere was this issue more apparent than in the "Stax Mafix" headphone community. Those high-end electrostatic amplifiers depend on some very specific high voltage transistors. I remember one particular excellent design, the KGSShv, went through many iterations in only a FEW years: transistors would be discontinued and then replaced with another part #, which then also became discontinued and subsequently replaced. I've heard five KGSShv builds and EVERY ONE sounded significantly different. Mainly because of the transistors. Those KGSShv amp versions ran the full gamut from very warm to very bright, and it could absolutely be pinned down to the big HV transistor in use.
You hear about SS & hybrid amp makers stockpiling silicon all the time - it's a necessity because of this fact. I'm sure Justin Wilson of HeadAmp has significant stockpiles (his BHSE actually uses a lot of this HV silicon). This almost certainly adds to the cost.
With tubes, pretty much all the common audio types are still in production. Even in the vintage tube market, there's PLENTY to choose from, since hifi audio mostly focuses on only maybe a dozen or so tube types (all which were produced in HUGE quantities), as opposed to how specific and small-batch the "good" transistors are.
Really nothing is more future proof than a P2P wired tube amp (with circuit diagram).