[avian]

I think solid state technology came too soon for the miniaturization of vacuum tube technology to be fully explored. Ken mentions in one of his older posts that even in these old tube-based IBM computers a lot of digital logic was in fact already done with semiconductor diodes, not vacuum tubes. Optimizations like the nuvistors never took off because semiconductors were already more economic.

I guess you could do a whole circuit on photo-etched or stamped sheet metal inside a glass envelope in a similar process that VFDs, although I don't know how you would implement cathode heating. It would be an interesting exercise to think where that could lead you, but I think modern VFDs date much later, when semiconductors already wholly replaced tubes in digital logic.

What fails in a tube is usually the heater filament, which is a wear part with a limited life. Where some (very) expensive tubes did go is to have replaceable heaters - so not more but less integration.