What I understand from this is that Roblox demonstrates large earning potential. A million is definitley no small amount, that's for sure. But we have to remember that there is (in any games market) a large gap between the earnings of select popular games we see on front pages and the earnings of a typical small developer.
Here is a table demonstrating this principle from this article: https://medium.com/@sm_app_intel/a-bunch-of-average-app-revenue-data-and-why-you-should-ignore-it-2bea283d37fc
This table displays in-app earnings from a single day for different Action games on both the App Store and Google Play Store. This demonstrates the descrepancy between the big titles on the front pages and what most people actually publish.
Here is also a GDC talk by Mike Rose talking about this same phenomenon on Steam:
The moral of these grim statistics is this: you absolutely MUST NOT assume you will find success based on the success of popular titles.
I expect that with so many games in Roblox's registry, the statistics follow a similar trend to this. As far as I'm concerned there is no escaping the fact that succeeding in any games market is actually much harder than making a good game in the first place.
I do not say these things to try to discourage you or tell you that you won't make money or find success. I myself have not released a commercial game, but I've attempted it many times and continue to despite weak financial prospects. I say these things to help you to keep your expectations realistic. I'm going to go out on a limb and say that there is a 99% chance that you won't make anywhere near as much money as Adopt Me on Roblox. Of course, that's also true for Clash of Clans on mobile or PUBG on Steam.
When you are learning to make games and program, I think that Roblox is a good way to start. However, for a financial endeavor, I'm much more pessimistic about that platform for other reasons.
For example, if you create a game in the Roblox platform, it's stuck there. Your game is exclusively available to only Roblox players. A Roblox developer is also at the mercy of Roblox themselves. If Roblox makes a change that hurts a developer or a game, there's not much that can be done about it. That principle, of course, still applies to other markets. Apple for example is removing Epic products from the App Store. Other than legal squabbles, there's nothing they can do about it. BUT: Developing in an environment separate from the markets you intend to sell in gives you flexibility and longer customer reach because your game is yours and you can do whatever you want with it. Sure, Epic may be hurt by that move, but it has other avenues to sell their games and make money. On the other hand, if a Roblox developer were too, say, get banned or something from the platform, then POOF. All of their work is gone.
I've only worked in Godot a little bit, so I don't know much first-hand about whether it's a good engine to work in. I do know that many say it's great and I don't doubt them. I would personally suggest that if your intention is to earn some money to go with Godot because then everything about your game will belong to you and not Roblox. You can also sell your work in multiple places. Itch.io can be monetized, and I know that you can put a finished game on Steam when you meet some requirements and pay a 100$ fee to sell. If Godot can port to things like iOS and Android, that's 2 more potential avenues if your game can use an appropriate control scheme. Of course, even if you do sell on all of these platforms, you still probably won't make much money.
But, even if you make 1 single cent, you'll still be miles ahead of many others because you made money by making a video game that's yours.
Don't forget as well that if it doesn't work out the way you wanted it to, it doesn't mean that it was a waste of time or effort. Failure is success in progress. By making any financial game you'll learn so, soo much that you will take with you to the next attempt regardless of where you make it.