I didn't see the manual posted, so I think this is it.
http://cp.literature.agilent.com/litweb/pdf/01740-90909.pdf
Those are good scopes, it is no wonder they are still working going on forty years later.
I have my doubts those LCD all digital scopes we have around us today will have the same longevity.
Any predictions on failure modes? Apart from electrolytic caps I would think they could last a long time, the LCDs are not as fragile or as consumable as a CRT are they?
From my mobile radio experience LCD displays degrade in the sunlight. While interior lighting is less destructive florescent light does contain some UV. Plastics age with heat nad time, and this is an issue with all electronics, again drawing from my two way radio experience flex cables will de-laminate over time. Some of the newer flex cable designs are better than the ones made even a decade ago they still are made of plastics and will fail sooner than other connection means.
pots can be cleaned and repaired, some rotery encoders like the ones on some of my amateur radio gear are repairable others not so much, and that may not be a big deal in the decades to come, they could actually outlast pots.
As far as rotary switches go, HP and others have done a great deal to eliminate the conventional rotary switches which require some degree of skill to clean, just spraying crap on one of those old pheonlic rotary switches will do more damage over the long term, when they get wet they swell up, the rivets holding the contacts loosen up and you have intermittent contact. You can see what HP has done in this scope to make rotery switches more reliable.
Over the decades silicon has replaced a lot of discreet components including switches and pots, this is not necessarily a bad thing, but it has its drawbacks. Fifty years ago the mainstay scopes were scopes like the HP-170 and the Tektronix-545. If you have one of those today, you can find replacement parts for just about everything in that scope without robbing parts from another 170 or 545. Each successive generation that fraction of specialized parts has grown The scope mentioned in this thread is on a par with a Tek-465. You can argue til the cows come home how reliable those custome chips are used in both scopes but when one dies, you cannot go to Mouser and buy one, you have to rob it from another 1740 or 465. Sadly this trend toward custom chips is all the rage in test equipment, add to that all that firmware. How do you back up that firmware that runs a modern digital scope? We haven't even touched on the subject of calibration, if the scope in question relies on an external means to access "soft pots" in order to make adjustments the failure of that must also be considered, when was the last time you saw a screwdriver, or a hex head alignment tool fail?
We have test equipment with enormous power at our disposal now, the new stuff is truly great in terms of preformance and in some cases makes gear of a generation ago look like junk. Consider the full price that goes with having that gear, how many here have a Rigol service manual for their scope?
Sadly our whole civilization is going this way Figuratively speaking all our gear will end up in a landfill (or recycled as raw materials) sooner or later. The trend in consumer electronics is that amount of time before end of life is shorter and shorter, in many cases it is due to fashion trends, in others it is because that TV or cell phone uses a chip that someone once made and no longer does cost of repair being factored out; the trend in how we design gear of all types is toward less repairability while the level o9f reliability remains fairly constant.
That ends with a shortened service life for the item in question.