I am a big fan of Bazel and have explored Nix (although, regrettably not used it in anger quite yet) - both seem like good steps in the right direction and something I would love to see more usage/evolution of. However, it is important to recognize that these tools have a steep learning curve and require deep knowledge in more than one aspect in order to be used effectively/at all.

Speed of development and development experience are not metrics to be minimized/discarded lightly. If you were to start a company/product/project tomorrow, a lot of the things you want to be doing in the beginning are not related to these tools. You probably, most of the time, want to be exploring your solution space. Creating a development and CI/CD environment that can fully take advantage of these tools capabilities (like hermeticity and reproducibility) is not straightforward - in most cases setting up, scaling and maintaining these often requires a whole team with knowledge that most developers won't have. You don't want to gatekeep the writing of new software behind such requirements. But I do agree that the default should be closer to this, than what we have today. How we get there - now that is the million dollar question.

Back in the days of Makefilea and autoconf, we tended to require specific versions and would document that in the readme.

Unless you audit the version you're pinning, what's the difference?