I thought the "instant" is for fast integration and editing, not for fast installation. But in reality, installing through git/composer is faster as there isn't anything to manually upload to the web server.
To be honest, I don't really see any issues with using command line tools like composer. Most of the PHP community has shifted to using composer and after all it's just a package manager - once all dependencies are resolved and downloaded, they can be zipped up and distributed as ordinary zip file - meaning we could have separate branch with all files (including files downloaded by composer) required to use the software.
One other great feature of composer is it's forcing usage of PSR loading standards, meaning every library that is on composer can be used in every composer-based project without any problems, which is giving great possibilities on extending the software. For example, if application is developed with Laravel, but you find a useful non-Laravel component on composer, you can still use it in your Laravel project basically with zero alterations.
Same thing goes for Laravel's artisan command line utility - every artisan command can be triggered from within PHP code.
The only problem is time, as it would require decent amount of time to rewrite everything from scratch.
But I'm still working on plugin system for current Instant Update setup. It won't be great (as it would be with modern frameworks ) but it'll do its job. Speaking of it, I could go and try to integrate some hooks!