The last paragraph's argument.
First, it makes no sense.
Second, it isn't true.
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First:
"because nobody wants to write 60 fucking pages of documentation on how to install"
You either target platform, or you target language.
If you target platform, you write.
On Mac: brew install [yourpackage]
On .deb distros: apt-get [yourpackage]
And you do the same with dnf, yum.
On Windows: [Here is your .exe / .msi]
if you target language, you write:
pip3 install [yourpackage]
composer install [yourpackage]
...
Generally, if you have time to write a shell file, you have time to say how to install it, because it is in the source code anyway.
It's fucking 60 times simpler to just write brew install [whatever].
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Second:
docker and nodesource does the thing you describe. That's it.
helm, bun, volta, starship download release archive or binary and expand it to target directory.
rustup downloads 2nd level installer and runs it.
nvm clones repo or load scripts and updates shell profiles
homebrew clones its git-managed tree, creates symlinks, etc.
k3s installs a binary, etc.
deno loads deno, and runs remote code over deno run -A --reload jsr:deno/installer-shell-setup/bundled
SKMan has a complex installer.
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There is a good reason that many of those packages are also installable over homebrew, apt, yum. Serious people will use them. And I suppose, in the future, all of them will be installable in a declarative way. Ops are going this way ... for ages ... since Ansible, maybe even longer.