Can I use WD40 and generic bicycle oil to maintain my MTB drivetrain? For years I've been using WD40, a brass brush, a rag, and some generic bicycle chain oil. My chain has no visible damage, and I have no problems with it. This is for a somewhat cheap MTB that sees everyday use for commuting.

What oil? I don't even know, and I don't believe it comes with any specifications. It is the absolutely cheapest I could find on eBay, it costs less than 1 USD including shipping, for 50 ml.

Is there any problem with doing this? If so, what is the problem? It seems to work great for me, contrary to what seems to be popular belief and advice found in professional (or not so professional, I have no idea who actually knows what they're talking about) maintenance videos?

The steps outlined e.g. here 
                
             are incredibly elaborate, they involve among other things, using a modified pair of nailbrushes to sandwich the chain, just to make sure that it gets cleaned properly. It also stresses how important it is to degrease it first. It also mentioned how you must use hot soapy water to clean it. It also suggests using aerosol lube before applying the actual chain lubrication. It is mentioned how you should use grease, and is this the shot where he painstakingly applies it to each link of the chain, by hand? Finally, it says it is not essential (which means, it is a really good idea to do it, you just don't have to) to dismantle your drivechain every time you want to clean it. I don't do any of these things, at all.

I use a worn brush and just rather carelessly scrub a bit at it, removing obviously large chunks and lager pieces of debris. Then soak it in WD40, wipe it off with a rag, use copious amounts of the cheap oil and then off the excess again. I've never had any issues. This is also where it gets to way below zero, there is a lot of salt on the road, often a lot of rain. The bicycle is stored indoors most of the time.

This MTB has some cheap Shimano parts, Shimano M190 31.8, Shimano M191 42-34-24 CG, Shimano CS-HG30-8 11-32, and the chain is a KMC Z72.

Trying to improve my own knowledge about bicycles, particularly in care and maintenance, seems to quickly make me run into a wall of enthusiasts that are just on a completely different level. It is rather demotivating. If there is indeed something wrong with what I do, it would be nice to know exactly what the problem is, and what I can realistically do to improve it. Because, doing what is suggested in that video, I think, is completely unrealistic to almost everyone except enthusiasts.
You're basically using WD40 as degreaser (it's a very thin oil that will dissolve away thicker oily gunk) and then using chain lube as chain lube. This is fine, as shown by the fact that you've been doing this for ages and not had any problems.

The only thing I'd suggest is that there's no need to use "copious amounts" of chain lube. It doesn't take long at all to just apply a dot of lube to each link of the chain as thown in the video, then let it soak in for a  couple of minutes and wipe off the excess.  Alternatively, hold the nozzle of the lube bottle against the chain at one end of the rollers and use the pedals to rotate the chain once round, while dispensing a thin stream of oil, then repeat for the other end of the rollers. If your chain has a quick link or similar connector, you can use that to see when you've been round once. Ultimately, though, since you're wiping off the excess, the only question is whether you prefer to use less lube or less of your precious time.