If with "all these extensions", you mean all validated extensions, then I agree and it is set up like this already. However, if with all you're implying not validated extensions, then it's a firm NO.
Why not to trust coders, creating a db of extensions where are all published, even if not validated, by titles and description, not into forums as it is now, where it is always possible to download the latest version?
Do you know that in some cms repository, all this is practically, after a first check about security, demanded to the code author?
And it run the most part of the internet sites.
phpBB should not close this possibility.
You want to endorse things that are NOT on top to you, and that you can clearly explain with an h1. You don't? Then you are limiting phpBB.
PS
AND please, make it easier to get phpBB notifications usernames , emails or any user info just adding a column that contain the poster uid