That's all true now but none of it was true back in the alpha days.

And once there were mods and mod loaders built on the obscured source, it became easier to not disrupt the toolchains than to bite the bullet; I think Mojang now wants to make moving mods easier (someone somewhere has to be a bit sad that there are famous modpacks running old versions of Minecraft because it's easier to backport everything to 1.7.10 (including running on newer Javas) than it is to update mods).