There seem to be a lot of strange fears from python developers about this approach and my comment on age was an attempt to assuage those fears. Since ruby developers are quite happy with bundler (at least in comparison to other communities) and have been for some time it's a reasonable point and not the only one I made.
I'm not sure how pip plans to use the lock file for projects distributed via pip. They may very well have a solution planned for that.
However, for projects that are distributed by source (which are many) the lock file ensures deterministic builds, and requirements.txt generally doesn't without being pedantic with your versions.