How well do things based IC based electronics last over time? If you put a collectible PC, music synthesizer, audio amplifier, electronic guitar etc. into dry, cool storage is it likely it will it play/boot up 25, 50, 100, or 500 years later assuming it has the right power source?
Most video arcade machines from the 70’s need to have capacitors replaced in the video monitor, power supply and circuit boards. If they haven’t failed already, they will, and that can cause additional damage.
The caps are the main issue with a lot of things. Sometimes a resistor will go wonky and eventually fry, often taking something else with it. But even those are sometimes caused by a bad cap.
Bad caps are a serious problem in recent electronics. Something from ~30 years ago has a better chance of lasting than something from 5 years ago.
The voltage things operate at plays a role. If there are key components that operate at 100+ volts, they are more likely to need work. Stuff that runs on 3-5 volts less so. Related to this is heat. If it put out a lot of heat, the damage comes on fast.
Guitars are fairly simple, don’t have much that can go wrong, and have easily replace parts. Few care about replacement of the electronics of those. Synths are far more complicated and harder to repair.
A few years ago I bought one of those little old Sinclair computers. It worked just fine and eventually gave it away. There are simulators that recreate the experience with hardly any hassle so it seemed pointless to me to keep it around.
Caps are the Black Plague of electronics. Either they dry out with the simple passage of time, or they’re among the billions of counterfeit electrolytics that have been causing failures in everything from PC motherboards to air conditioners.
At least most of the counterfeits are easy to spot as their cans will have either ruptured very obviously or they’re bulging. Dried-out caps are less obvious and unless you re-cap the device, you may not know they’re bad until they explode.
Something else to watch for is tin whiskers. These can destroy an IC internally, and there’s no way to inspect for them, or to repair them. They’ve been a problem since the 40s. NASA Goddard Tin Whisker Homepage
Anything with EPROMs or EEPROMS is subject to bit flip. Bits stored on the chips can spontaneously flip from 1/0 or 0/1 (read this is from anything from cosmic particles to simple earthly issues). A lot of early systems (and game cartridges) used these for low print runs, rather than ROMs; easy to fix if you’ve an EPROM programmer and don’t mind damaging labels by punching through with a screwdriver to get to the chip, and assuming a correct copy of the data exists out in the ether… But definitely affects collectibility. Affects small runs of CE, arcade units, cartridges.
Opacity of media and “rot” affects optical electronic media. Laserdiscs, CDs, etc can get corrosion of the metal layer, glues to hold layers can go bad, plastics shielding layers can change form, erasable/recordable layers can easily deform.
Traces can corrode, like in edge connectors and plain circuit boards. Solder points can go brittle and break. If it’s a surface-mount component, good luck.
CRTs in TVs and arcad games, as well as plasma displays can be exhibit burn-in of stationary images. Pac-Man attract mode on my 1942 arcade? Sure!
Magnetic media flakes out and goes bad pretty quickly. Already difficult to source working discs for 90s computers… How about the 8-track played on loop in the arcade Journey: Escape unit? 8-inch games for old computers? Similarly for paper tapes for early computers? They crumble in paper tape readers many times, and were sparsely distributed to begin with.
Caps can go, maybe some ROM will get corrupted, maybe corrosion will set in, but in general the discreet components and ICs will last. I’ve been able to power up some minor electronic devices for at least 25 years. I have a simple multi-meter that’s at least 25 years old, the battery compartment door is lost and the sockets for the probes have expanded and don’t hold tight, but it still works. The more components in the device the more likely some failure will occur over time.
Passive electric guitars are the VW Beetles of instruments (unless you are swapping pickups on a semi-hollow - sigh). They should be maintainable and as long as their is something they can plug into, will be a viable tool for music.
Guitars with active electronics or guitar synths - honestly, who cares? 
Effects - well, these days some boutique stompbox guys make a living finding old-school chips that were used in classic effects, like old Tube Screamers or ProCo Rats. That suggests that the parts are scarce. But digital emulators are getting ever closer.
Amps - tubes will go away. Can someone “craft” vacuum tubes/valves by “hand” once volume manufacturing ends? I have no clue but assume not. Pity - playing a great old guitar and amp is more about playing the amp vs. the guitar. Can’t really replicate that with modeling amps, although in general, they are a fine alternative to standard-circuit amps.