Wildly unrealistic for many reasons, but I'll just focus on the legal ones. First, as you say, the Moon Treaty is not widely ratified. Those countries that HAVE ratified the Agreement are of course subject to its provisions. But lets set that aside since here we are assuming that the countries in question aren't parties to the Moon Treaty.
It is possible for provisions of a treaty to become applicable even to non-parties if over time the international community comes to recognize them as having passed into customary international law. BUT there are very specific criteria for that, as well as some caveats. First, the criteria. To find out whether a specific rule is part of customary international law (CIL) you have to look at two things. (1) Practice. Do a wide variety of states do or refrain from doing the things stated in the rule you are investigating? You need broad adherence to the rule. (2) Opinio juris, which basically means "belief in legality". That means that not only do many countries need to be following the rule, THEY MUST SPECIFICALLY SAY THEY ARE FOLLOWING IT BECAUSE THEY BELIEVE IT TO BE LEGALLY REQUIRED. That's a very high bar, and the Moon Treaty absolutely does not pass it. I can't think of a single non-party to the Treaty that has ever said that any provision of the Treaty is legally binding on non-parties.
So that answers the question. A few additional notes for completeness. First, treaties don't pass into CIL as a whole all at once. You look at individual provisions and do the inquiry one by one. For example, with regard to the Outer Space Treaty, Article I about freedom of access to space has almost certainly passed into CIL, but many other articles (say on emergency assistance) likely have not.
Finally, EVEN IF A RULE IS CIL, individual countries can still opt out if they have "persistently objected" to the formation of that customary rule. This is why you see countries like the United States repeatedly say negative things about the Moon Treaty, for example, JUST IN CASE the international community ever decides that portions of it have become customary, because even then, the United States would have persistent objector status and not be bound by those customary rules.
So that's the legal answer. There are of course all sorts of enforcement problems as well, but those are for another discussion.