(I don't have an answer, fwiw, but I'm not overly keen on this solution)
1) Children are already restricted from consuming alcohol and coffee.
2) You can only drink so much coffee before your body asks you to stop, or gives up completely. It's not pleasant.
3) Coffee doesn't have this level of addictive properties (alcohol has, and as you'll note, alcoholism is a big problem, though see #5 and #6).
4) There are already "social antibodies" preventing most people from overconsumption of alcohol (and to some extent, coffee).
5) Alcohol prohibition didn't work because it unfairly punished the addiction-free majority, but what's proposed here isn't a restriction on video games, but on a particular aspect of a particular set of them. The proposal doesn't restrict the fun-giving part, only the addiction-inducing and addiction-exploiting parts.
6) Nobody designs alcohol or coffee with the sole intent of causing addiction their users that can be exploited for money. Whereas these games absolutely are designed like this.
However I still disagree that putting a $50 (for arguments sake) cap on the amount to be spent will even help. If that issue worked, it would work for alcohol too. Just because alcohol makes you ill doesn't stop people with an addiction from drinking themselves sick and waking up and immediately resuming drinking. Coffee may have been a bad example but tobacco is another reasonable comparison - limiting tobacco consumption to an arbitrary limit won't help people with a nicotine addiction.
There are many games out there (world of Warcraft for example) which impose weekly lockouts - no matter how much you play you can't progress past a certain point. What ends yo happening is people start at the lockout, burn through the allotted content as soon as possible, and then just wait a week and come back. Implementing a limit isn't going to stop people who are addicted to these games from finding 5-6-7 of them to make progress on to scratch that itch.
To address one of your claims directly:
> The proposal doesn't restrict the fun-giving part, only the addiction-inducing and addiction-exploiting parts.
Firstly how do you define which parts are fun giving and which parts are addiction inducing? There is a very fine line between engaging and enabling, and it's a fuzzy one. At their core, the majority of games are Skinner boxes, and everything else is dressing over that (even MP games). What is firmly in the camp of fun for many many people could easily be considered enabling for a large group of others. People enjoy progression systems in games, they are engaging and fun, and there are clear cases where these are abusive. I don't believe that saying "if you can get 20x more people addicted you can still earn the same" is the right way to punish those abuses though.
We should look at ways of reducing the reasons why people fall I to these traps, and look at regulating the monetisation methods of these games, otherwise they'll just change tack and appear in a slightly different form in 18 months time with creative workarounds (which have already been mentioned - multiple games with tie ins, gifting, streaks, external purchased to interact with the game)
As for the "fine line between engaging and enabling", two points. One, it's not just "enabling". Selling tobacco is "enabling", those games are actively designed to max out the addiction factor. Two, I agree the line is fuzzy, but I think the "you'll know it when you'll see it" argument does apply in this case. There is a meaningful difference between games optimized for being fun and an artistic expression - like most video games in 80s, 90s and 2000s - and skinner boxes thinly wrapped in a theme. It's hard to define what that difference is, because it ultimately is a difference of Colour[0], which is something hard to capture in laws. But I believe that we must try to discourage people from creating "games" of the "Skinner-box" colour.
To give random examples of games I spent ridiculous amount of time on as a kid: StarCraft and Fallout 1 and 2. They were "addictive" in the sense a good book or a good sports can be addictive. But they were first and foremost designed to be fun and tell a story, addictiveness was incidental. At no point they tried to use it to get more money from me/my parents.
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In this case, large companies with multiple games can have cross game promotions. Also, $50/game/month is still a very larger amount of money for many people and doesn't resolve the issue of kids being allowed to gamble.
The business model for these games is that 99% of players spend negligible amounts, and 1% spend hundreds or thousands; the minnows are a loss leader to get to the whales. Restricting spending to $50 or $100/month would destroy games like this entirely. (Which I'm fine with, personally.)
Also, are the minnows really loss leaders or just not as profitable?