Notch once promised to make Minecraft open source "once sales start dying"[0] but he received 2.5 billion reasons to let that decision be taken off his hands.
I've tried Minetest a few times. My issue with it is that it just feels very unpolished. Things like the lack of head bobbing, so it feels like the character is gliding across the ground, a pretty bad looking block break animation without enough particles, a very floaty-feeling jump, and on macOS, the scrolling in menus is completely broken and it's not running at the display's native resolution (no HiDPI/"Retina" support), you can't change settings in-game, hitting escape in the main menu just exits the program, etc etc etc. It feels like a game engine demo project, not a game. Everything down to the name "Minetest" feels unfinished and unpolished.
This is exactly the mind-set which prevents projects like Minetest from becoming "real" games. Mods are great, but the core game has to be good by itself to attract people. And if Minetest doesn't want to be a "real" game which is good out of the box, that's fine, being a game engine playground is perfectly fine, but that means it's not even trying to be a Minecraft alternative.
The state of open source Minecraft is nuanced enough to be interesting. The Minecraft ip and codebases are currently owned by Microsoft and have technically always been closed source, but the nature of the game and its development history are such that third party mods and modding apis are very common. Because the original edition of the game is written in Java, it's relatively easy to decompile and develop for. In fact, Mojang Studios has been including a deobfuscation map with each snapshot release of the game for a few years now which would allow you to decompile the Java edition of the game and use the same names that they use internally [0]. Though, most modders prefer to use the mappings provided by modding apis like forge and fabric due to tradition and licensing conflicts. Depending on your definition of Minecraft, there's also plenty of open source implementations of Minecraft Classic, a much older version of the game that's easy to reimplement but apparently still has an active community [1]. Then of course there's Minetest, an independent totally open source voxel game that happens to have a lot in common with Minecraft [2].
I should probably also note that there's something of a conflict in this area surrounding the other edition of Minecraft, Bedrock Edition, which is written in C++ and therefore difficult to decompile and mod. Bedrock Edition is much more closed than Java edition in a number of ways, such as including a store built into the launcher for buying maps, texture packs and skins whereas these would need to retrieved externally (usually for free) on Java edition. However, Bedrock Edition does have a first party modding api, a more feature rich internal scripting system with the concept of "behavior packs" [3], and has led to the development of a number of tools used by mod authors for both editions, such as Blockbench [4]. Ultimately Java Edition and its community have the legacy of Notch and Mojang with a long history of community contribution while Bedrock Edition was only developed after the Microsoft acquisition and is much more in Microsoft's style.
EDIT: Turns out the Bedrock Edition modding api was discontinued earlier this year [5].
1. Community oldtimers and loyalists really like Java.
2. Bedrock has no Linux port.
3. Java is the lead version - new features are added to it first and then backported to Bedrock.
While #1 and #2 could be written off as "costs of doing business", the latter is a significant problem. The current creative process that Microsoft and Mojang has adopted is that Mojang implements new features and versions in Java first, and then another team in Washington reimplements them in C++ for Bedrock. Deprecating Java means making everyone at Mojang switch development tools and languages and adopt an entirely different codebase. It would be just as disruptive for Mojang as it would be for modders.
I don't disagree with your conclusion, but I do think your information is a bit outdated. In recent updates, effort has been made to keep both editions as much in sync as possible with explicit goals of feature parity and synchronous releases. I don't work at Mojang so I can't speak with complete certainty, but I know there are a number of developers who develop features for both editions simultaneously [0]. That said, all the core developers I've heard from are extremely passionate about the Java edition of the game and have absolutely no plans to stop supporting it. If that ever happens, it will have been a Microsoft decision that they had no say in.
Unfortunately sifting through the last few years of Minecraft.net articles and Minecraft Live footage to find sources isn't a great use of my time right now, but I'm sure it's out there. Bedrock was shifted to the same version numbering system as Java in the last year iirc and they've been releasing snapshots and betas with the same content on the same day for all of the 1.19.3/1.20 snapshots so far and I think the 1.19.0 ones as well.
