Sure, there is some survivorship bias involved (for example, most HP 8xxx pulse generators were garbage IMO) but with the throwaway instruments they make nowadays, hardly anything will be left in 40 years.
Either because configuration from PLD or microcontroller EPROMs leaks empty and no way to recover. Or it looses calibration and no public info on how to calibrate. Without schematics, even something simple can fail and you can't repair, because you don't realize it's broken, without the insights a good schematic provides. So why no schematics, service and calibration info now, if I pay 10x the price of a chinese instrument?
The better scopes/logic analyzers back then were all modular, Tek 7k, 11k, HP 54750..., makes an instrument much more versatile and it's simple to replace the frontend, once something breaks down. Makes diagnostics simpler as well, than the 1board garbage we have nowadays. Why not today, make 1 good instrument with different plugins instead of 10, each artificially crippled in a different way? And split things up onto different boards, so things can be tested by swapping them. Once a moderately complex board is identified as broken, parts level repair usually isn't that hard, in case the manufacturer would provide the parts.