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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

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.

You might have a slightly shorter chain life, and slightly more chain drag than if you followed the recommended chain cleaning procedure and used a better lube.

FYI if I remember correctly there is a GCN video where they show a quick chain cleaning method using WD40. WD40 is lube in a solvent, so it works to remove old lube and contaminants then evaporates so you don't have to wash it off.

You might consider a better chain lube. It's more expensive but if you apply a drop per chain pin as in the video a bottle lasts a very long time.

I think you're doing good job. I think you can save time by using a chain cleaner tool, I find it satisfying to watch the chain becoming clean just by turning the crank by hand. You can buy the solvent or just use soap water.

After degreasing, I also use WD-40, but it's WD-40 Chain Lubricant. It's probably the same as other lubes. The advantage with WD-40 is, again, I just turn the crank and spray. Because of the fine spray, there's less drips on the floor.

I wouldn't worry about damaging the chain, it will worn out before anything. I replace mine every 10,000 km (really stretching it here)

Your system is fine. You can pay a bunch of money for Friction Facts and find out how many watts you're losing by not doing what the pro's' mechanics are. (Ha.) But that doesn't matter. You keep your chain relatively clean and lightly lubed, and there's not much more that needs to be done. Any alternative should be chosen only if it makes your life easier.

I used to use Tri-flow because it lasted longer than WD-40, but it was similar. However, when I moved and began riding in dustier conditions, this thin, wet lube attracted so much dirt that maintenance became a nightmare and I was always full of black gunk. I eventually switched to hot-waxing the chains and putting DIY wax (same batch, just dissolved in paint thinner) on as a touch-up. I have a cheap ultrasonic cleaner for parts cleaning and it works well on chains too, so it's not much of a problem for me, and I stay a lot cleaner. This is way more work than most people would want to do, but I don't have to deal with the gunk on me or the bikes, so it's a net gain for me. Wax ain't great in wet environments, though, as it doesn't keep the rust at bay. I'd go back to tri-flow if I were in the rain more.

If your system works for what you want, stick with it. I'm also down on sprays because it's really easy to get them on disc brake rotors and pads. Drips tend to go only one direction.

Yes, you can use WD40 and generic oil for maintance. But also you can use diesel fuel and two old tooth burshes linked together with ducktape. Diesel is cheaper but if has few drawbacks. It can cause cancer if you don't protect your skin(with gloves) and after cleaning chain you should wash chain with soapy water.
I use diesel for over a year and chain always looks shiny.