Demagnetization only really effects tapes... and AFAIK there aren't any major consoles that use tapes
Most consoles will last longer than you do if you don't use them and you keep them in a climate controlled environment (read: your house, not some swampy or freezing garage). Humidity is important in the long run too, too dry and paper will become cracked and brittle, to too damp and paper will get soggy and warped and metal will rust (this includes the electronics inside the devices as well)
It should go without saying but also make sure you keep the consoles and boxes away from areas where they'd be effected by cigarette or other kinds of unnatural air pollutants, and keep them away from direct sunlight (this, given enough time, will fade the print on boxes and change the color of plastic).
The best thing you can do is find ways to seal items... seal them when they're in an ideal environment and leave them sealed and you wont have to worry about a lot of the above problems.
Beyond that each consoles has it's own unique problems that are effected by time.
old cartridge based systems have internal batteries for saving games these tend to die eventually, Hard drives will generally last a very long time as long as you don't drop them or use them... laser arrays and mechanical parts (like disc trays, springs, and gears) will generally be fine as long as you don't use them)
in many of these instances repairs can be made to fix the items... hard drives batteries and springs can be replaced with newer non-original hard drives batteries and springs. finding replacement laser arrays however is nigh impossible, you'd have to hope to find another console with one that is still in good condition.
After all the above the next thing to break would likely be capacitors in the power supply (or for the screen if we're talking mobile devices) these can be replaced if you know what you're doing with electronics, but again it's only a problem if you're using the device.
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If you're a serious collector and collecting for profit you'll only buy things that still have the factory seal around them, and in that case who cares what the condition of the items inside is like, as long as the seal is good and the box looks good the item will be considered "flawless".
Unless the item is unbelievably rare, or when it was new it was never shipped from the factory, then you should only ever buy things that still have the factory seal... in the instances where it came unsealed or you find something that is rare and pristine but unsealed, you should make sure everything is in good shape and then seal it yourself and treat it like a sealed item from then on out.
If you're collecting for your own enjoyment and to use these items then buying unsealed is fine... if you want the item to increase in value then don't waste your money on anything but factory sealed items.