The point is that if I'm writing a library and I specify `requests == 1.2.3`, then what are you going to do in your application if you need both my library and `requests == 1.2.4`?
This is why libraries should not use lockfiles, they should be written to safely use as wide a range of dependencies' versions as possible.
It's the developers of an application who should use a lockfile to lock transitive dependencies.
The lock file is for developers of the library, not consumers. Consumers just use the library’s dependency specification and then resolve their own dependency closure and then generate a lock file for that. If you, as a library developer, want to test against multiple versions of your dependencies, there are other tools for that. It doesn’t make lock files a bad idea in general.
As another library developer, of course I want to test against multiple versions. Or more accurately, I don't want to prevent my users from using different versions prematurely. My default expectation is that my code will work with a wide range of those versions, and if it doesn't I'll know - because I like to pay attention to other libraries' deprecations, just as I'd hope for my users to pay attention to mine.
Lockfiles aren't helpful to me here because the entire point is not to be dependent upon specific versions. I actively want to go through the cycle of updating my development environment on a whim, finding that everything breaks, doing the research etc. - because that's how I find out what my version requirements actually are, so that I can properly record them in my own project metadata. And if it turns out that my requirements are narrow, that's a cue to rethink how I use the dependency, so that I can broaden them.
If I had a working environment and didn't want to risk breaking it right at the moment, I could just not upgrade it.
If my requirements were complex enough to motivate explicitly testing against a matrix of dependency versions, using one of those "other tools", I'd do that instead. But neither way do I see any real gain, as a library developer, from a lock file.
This is why libraries should not use lockfiles, they should be written to safely use as wide a range of dependencies' versions as possible.
It's the developers of an application who should use a lockfile to lock transitive dependencies.