If my memory serves, the stated justification for not going open source was copyright and trademark protection. Apparently, that is no longer a concern, if it ever really was.
Now I'm bracing for them to drop support for Java Edition entirely and go strictly Bedrock in a couple of years.
Perhaps Minecraft 2.0 is finally nearing release.
Actually, there was technically a Minecraft 2.0 release, but it was an april fools prank in 2013.
Relevant wiki link: https://minecraft.wiki/w/Java_Edition_2.0
I don't see that happening any time soon. Too many people at Mojang care about the Minecraft Java Edition community too much.