No, it wasn't me that modified the forks with a drain plug.
FWIW, the oil condition in forks isn't anything like engine or transmission oil. Not a lot of exposure to elements (combustion products and water vapor) and not a lot of huge changes in temperature. The oil does not break down under use. About the only metal particles it sees are the very few from the bushings, which rarely have any wear at all in 100k miles. The grunge you see is the little bit of seal material that is worn as time goes on. It blackens the oil quickly so it may seem really dark in just a short period of time.
The only time I change fork oil is if the seals are leaking or if I have to have the forks apart for some other reason. And, as has been said above, fork seal problems are usually from external things, like bugs and dirt on the forks or pits in the chrome plating.
Off road bikes are another matter. They are exposed to a ton more dirt and water. Usually they are also a lot harder on the shocks.
And, yes, leaving a bike to sit for years means the fork seals will probably be toast. The seals are lubricated by the action of the forks. If they aren't exercised, the seals will dry out and get stiff, then leak. This is not as common as the old days as seal materials have improved so much. There is also the problem of small amounts of corrosion on the fork tubes when sitting. Then when first moved they rub against a dry seal.
Last, sunlight. The seals themselves are not exposed to the UV of the sun, but, those dust shields are. They are the first line of defense for the seals by 'wiping' the dirt and grit from the fork before it gets to the seals. The UV degrades them and then the seals are exposed to more harmful stuff over time.