If you require end users (and possibly libraries? IDK) to manually specify every transitive dependency of a dependency (but not hard-code/vendor it), this should act as a forcing function to reduce transitive dependency explosion in libraries (because it would degrade user experience). I'm not sure if users should have to update every dependency by hand (this discourages updates which can cause security bugs to persist, but automatic updates makes supply-chain attacks easier; AUR helpers generally diff PKGBUILDs before committing them, which partly protects against PKGBUILD but not source attacks, and even distros did not protect against the xz attack).
Another factor is that updating C++ compilers/stdlib tends to break older libraries; I'm not sure if this is any less the case in Rust (unclear? I mostly get trouble with C dependencies) or Python (old Numpy does not supply wheels for newer Python, and ruamel.yaml has some errors on newer Python: https://sourceforge.net/p/ruamel-yaml/tickets/476/).
This is optimizing for the wrong metric, IMO. If I look at the dependency tree of a fairly hefty project in rust, mostly what I see is the same amount of code as an equivalent project in C/C++, just split into multiple packages instead of bundled up into one source tree. Which ironically means packages tend to be able to pull in the minimal amount of excess code through transitive dependencies. All that you'll do with this kind of incentive is push packages into effectively vendoring their dependencies again.
To the best of my knowledge (I only dabble in Rust) there aren't often too many breaks unless code accidentally relied on soundness bugs which Rust makes 0 promise of retaining to keep code working.
Another factor is that updating C++ compilers/stdlib tends to break older libraries; I'm not sure if this is any less the case in Rust (unclear? I mostly get trouble with C dependencies) or Python (old Numpy does not supply wheels for newer Python, and ruamel.yaml has some errors on newer Python: https://sourceforge.net/p/ruamel-yaml/tickets/476/).