Hop degradation is the same as with anything. Heat, moisture, oxygen, light, physically breaking them up and so on. If they are in sealed bags and an inert atmosphere keep them in the fridge or better freezer. It looks like everybody bitters with magnum because it is a high alpha, reasonably priced, low aroma/clean bittering hop which stores well and as a result it is attractive to commercial brewers. Whenever I open a fresh pack I get a big hit of tangerine orange and I redundantly remark "this'd be nice as an aroma hop!" (it isn't, it needs to be boiled for at least 20m). For most recipes however there is nothing special about magnum and we'll sub it out for more economical bittering hops depending on the market conditions. I'd say it isn't much cop for a home brewer who might want 6g in a batch and as such have it kicking around forever. In fact with any decently high alpha 'rock star' hop (citra, mosaic, centennial, simcoe, el dorado, vic secret, galaxy) you might find you need so little to bitter with anyway that it is better to just use a few grams of that from later in the boil and save buying, opening and then having it kick around so long all your brews suffer for the next 3 years while you use it up.
There are times when I want specific hops because you need them, but there are times when a harvest fails you, prices shoot up and/or the profile changes and as such blending, the art of making the best with the ingredients available to you becomes part of the skill of brewing. Having an open mind and holistic approach to hopping where you take advantage of a glut of something is part of the charm of 'rustic farmhouse brewing' for me and I like to imagine fits in with the ethos of farmhouse brewing who'd have not sweated too fine details. Like if I want a 'noble' aroma hop, there are so many to choose from I might as well choose by price, year and intended purpose because they'll all be sort of alright, with the highest price and freshest harvest reserved for the aroma addition and the aged mash hops for <5IBU's rightfully for whatever is cheap and not known to be awful.