I cannot comment on the technical advantages of this approach, but I’ll share my limited experience with standalone package manager.
But first, a question: is IonCLI a proposal for replacing the package manager or an additional tool that packs additional functionality but still depends on the package manager?
When I decided to switch away from Matlab, not being a CS person, not being used to cli and working on Windows, I wanted to choose between Python and Julia. One of the reasons I chose Julia was the package manager built into the REPL, the ease of installing packages and activating environments.
I gave Python a try and went mad with pip being incompatible with whatever packages and all sorts of errors I couldn’t understand. This gets worse as different distributions have a slightly different workflow: conda, standalone install, pycharm. I found myself constantly switching between windows during installation of packages and creating environments, trying to understand which console to use or check for errors.
As a basic user I find it much more convenient to have this functionality available in the language itself, one key press away, than to switch to another window and do the same thing and worry about compatibility and all the usability quirks. I think the powerful REPL with built in package manager is one of the strong points of Julia.
Other languages don’t have such a powerful REPL and an external tool is their way of managing packages, but why steer this away from Julia?(Racket also does thia with raco and I can’t understand why; maybe because it was faster to do it that way?)
I’m sure the technical reasons you stated are sound and please don’t take my comment too harsh because as you can see I’m really biased against Python and pip.