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I don't really agree that "New users must look for a plugin manager and figure out how to install it". The only users who need to do that are the ones who have already found a plugin that they want to install, and which also doesn't provide installation instructions of its own. I don't agree that "Lack of a declarative way to specify plugins" is a valid problem, since this problem is actually introduced by bundling a plugin manager.
And this justification for creating the feature completely ignores the several high-quality existing, unbundled implementations of plugin managers. The installation of Lazy.nvim is entirely within your nvim init files, demonstrating that 3rd-party plugin managers can have simple installation instructions.
The bundled package manager also makes some trade-offs that 3rd-party ones don't have to make (e.g. support for non-git plugins). This is addressable, like the lack of lockfiles, automatic dependency management, and re-implementing every other feature that existing plugins already provide. That's why this is bloat. This task is better-served outside of the core project.