I think it simply took that long for a majority of the existing old test equipment to get sold through Ebay.
There should always be old test equipment though, companies are making new equipment constantly and as time marches on that equipment becomes old and in theory should end up on the surplus market. It may come in waves as new technology appears but I still would have thought there would be a steady supply.
I wouldn't count on that, and I'll guess the availability will be less in the future:
- instruments are not build like tanks any more, nobody designs lab instruments with survivability to a nuke attack in mind
- less repairable because of the lack of documentation and because of the lack of software/configuration tools
- we don't know yet how reliable will be a flash memory in 50 years from now, or a FRAM chip (that has a limited number of reads, too, so it will wear out simply by running)
- since the frenziness of data leakage there is a tendency from big companies to simply destroy the old instruments just to be sure no password or project info will leave the company together with the old instrument
- there is a fashion to recycle old objects at all cost, even when the old object still has users and usage
- big manufacturers of instruments realized their old instruments are competing over their own market of cheaper instruments
- there is a tendency to put unique IDs inside chips, to encrypt the firmware, to link everything with a call home or a unique registration, therefore re-flashing a modern instrument is almost impossible without software tools specific to that line of instruments and specific to that company
Therefore I'll say the abundance of instruments that are affordable, second-hand, but still gorgeous in performance (the kind of boat anchors we see today on ebay) is a unique feature of our times, something that was caused by the cold war era combined with a very specific level of technology.
The instruments back then were seen as standalone and self-contained objects. Now it is not like that any more. Now the tendency is to have a box with no screen and no buttons, box that is usually controlled remotely by a computer. So now the instrument-box suddenly will need a very complex environment around it, an external computer, with external software, and external operating system, and an external network to transfer data, and so on.
Good luck restoring and activating a Windows 10 in the year 2070, so to re-flash or simply to use that cool Tektronix spectrum analyzer box-instrument from 2020, with no buttons and no indicators on it!
With the current trend, I see everything will be closed and locked-down and encrypted, first to a certain brand and environment, and very soon (as in a few decades away from now) we will see everything (everything as in objects, services, etc.), even your faithful DMM or your faithful handgun, everything will require a personally encrypted key and a valid personally authorization, or else that object/service won't be available to you.
This will happen whether we like it or not, simply because the actual level of technology allows to have that.