Note that the goal is to recreate "old-school Minecraft", but actually even older school than you're thinking of. For the first year or two of the game there was no survival mode, only creative mode. And people played that version for much longer as it was available as a free to play in-browser version for like the first 5 years of Minecraft's life, even as survival mode got developed in the branch that led to one of the current versions. Also the main branch didn't have creative mode at the time (or even multiplayer for some of it), so some people played classic for creative mode or multiplayer too.

It's not like the way modern creative mode is just a game mode in the same engine as modern survival. Classic is quite a different game so what would a classic survival mode be? Either you're inventing rules for a classic survival mode that didn't work, or you're implementing an Indev/Infdev/Alpha clone in the same codebase as the Classic clone, and I think those two things are different enough that you'd still end up just maintaining a fork inside the same codebase with the amount of conditional "if(alpha) else if (classic)" behaviours you'd need. It's like trying to have the same codebase be classic mac os and os x at the same time. I can't blame a maintainer who doesn't want to sign up for that.

Fwiw re: lack of survival- mojang has tried (and backed off) to take down cc under the assumption it had crafting/survival. The lack of survival aspects is the only reason it’s still around.

That, and classic v .30 doesn’t have any survival features