"(e.g. tangentially Tektronix had a scope with synthetic sampling rates in the THz in the 60s or so), whereas modern stuff is becoming - even in simple designs - harder and harder to analyze as margins are being chased."

Yeah, yet another sad engineering story is modern documentation—or more precisely the almost complete lack thereof. One of the great things about Tektronix was that it was an electronics instrument company run by engineers—engineers who were truly committed to both technical excellence and the highest quality of manufacturing [at least that was my experience of the company back some decades ago when I was buying its CROs and analyzers—I'm unsure of the current situation].

Rigorously complying with technical standards was a Tektronix mantra—and that included supplying customers/users with top-notch, high quality documentation. Tektronix user and service manuals were legendary for not only being both comprehensive and clear to understand but also for their ergonomics—you could find any info needed about an instrument within seconds (the manuals were ergonomic design masterpieces).

Unlike most of today's equipment where most of the technical information about a device is considered proprietary and held locked up by corporations, Tektronix manuals provided the customer with just about everything a customer wanted to know: detailed technical descriptions of its electronics operation, clear and comprehensive circuit diagrams with layouts including parts cross references together with properly indexed parts lists for purchasing spares. [Right, back then, there was none of this current fight about the 'Right to Repair' as that was the assumed norm by both users and manufacturers alike - yes, we had the 'Right to Repair' and it was stolen from us.]

That Tektronix published such detailed information for everyone to read is proof that patents, copyrights and manufacturing trade secrets alone could protect the company's IP. It demonstrates the fact that it is unnecessary to hide almost everything about equipment except its bare operational details.

Tragically, the problem about the paucity of information about equipment coming from manufacturers these days has now been around so long that both younger users and engineers simply do not know what decent equipment documentation looks like or entails. For those who don't then I'd suggest you have a look by downloading the Tektronix 7834 oscilloscope manual from the Internet Archive (this is a device that I'm very familiar with):

https://archive.org/details/tektronix_7834

(Note: for a better quality presentation, download the PDF version without the embedded text; alternatively download both versions.)

INCIDENTALLY, NOTE THAT 'TROUBLESHOOTING', P-4.3; 'CORRECTIVE MAINTENANCE', P4.12; AND THE SPARE PARTS LIST, P7.1, ARE ALL COMPLETE AND WITHIN THIS ONE SINGLE MANUAL. IT'S PROOF THAT BACK IN THE 1980S AND BEFORE THAT THE 'RIGHT TO REPAIR' WAS AUTOMATICALLY ASSUMED BY EVERYBODY!

Finally, I'd add that today's practice of locking up equipment information behind corporations' firewalls—information that only a few decades ago was freely available from many tech manufacturers—has the negative effect of deskilling* the population. Not only this but also it wastes considerable time and effort as users have to try and discover (or reverse engineer) info that rightfully ought to be in equipment manuals (consider the total productivity loss when taken across the full population of users).

* For example, Tektronix manuals were wonderful training aids for those wishing to understand how an oscilloscope works. With the aid of a second oscilloscope and the manual of the first, one, say, could trace the passage of signal from the vertical input amplifier to the defection plates, etc., etc.