I'm not an expert in economics, no. Why should I? This is the basic logic.
G2A is a reseller. If you resell, you must acquire the key. If you acquire the key, you must buy it from the dev. If you buy it from the dev, they make money. Now, the question is how much they actually make money and it's pretty obvious that they're not making as much as the 1st class regions usually generate. The reason why G2A's prices are low is from two kinds of people. Some are in a hurry, so it's just a fire sale for them, since they don't want to pay for the market spot anymore or something like that. The others buy from different regions. That's the one where the dev won't get as much profit.

Now, I like how you're talking about how they all deserve a livable wage and whatnot and in reality, no, in most cases they don't. If you make a good product, you'll be successful. Take for example games like Boogeyman, the dev, who I actually know as well. This was his first game. he deliberately hasn't generated keys for it yet and he sold over 10,000 copies. He lives off of that and now that that source of income is drying up, he'll either make a sale, generate keys for 3rd party sellers, or talk with bundle sites about having it in a bundle. And, if that doesn't work, which is possible, he'll.... get a job (wow, what a concept! That's a surprise. Innovative! Wow!).

Now, I know you'll say: "Oh, but a lot of games can be good, but they'll just get buried under the taff that Steam has on its store!". Well, here's an amazingly simple answer to this... Advertise. Advertising is easier and cheaper than ever before. Free, in fact. All you have to do is make a fan page, advertise it on other places, tell your friends for word-of-mouth, make giveaways on sites like SG and so on. Also, you can give it to Youtubers and streamers.
Streamers and youtubers constantly talk about how to attract their attention. You just have to do some research about it.

And also... if they want a liveable wage, then they really should get a job. If you quit your livelyhood to make video games, a risk that's already huge, even if you don't quit your job, then I must say that you are insane. They have barely any experience and no financial backing. While this could be spinned around by saying "Well, help him then, he needs all the money he can get", then I'll just answer by saying no, since stupidity shouldn't be rewarded. I won't give out more money on the sole basis that he made a dumb decision.

So, yeah, people who buy from G2A, do pay for their work. It's just that they already got paid in advance.