If you shut down your extension and they had to put up their own copy, they'd have to re-acquire your installed base. That could be a sharp decline in value to them, particularly if the extension mostly got popular off a one-time front-page feature rather than via gradual discovery with active word of mouth.
The chance that people jump through all the hoops to impulse-install again twice is low. They'd have to really like your extension, even if your version notified them of shutdown of yours and availability of the new one. Growing an installed base is generally more a factor of not chasing your users away than explicitly doing things to retain them. That change would chase them away.
In an ideal world, you'd be able to officially transfer the single extension to a new owner while keeping all the installed users--preferably with a notice dialog enforced by the browser popping up to tell the user the ownership changed and offering them a chance to uninstall. That would also chase some users away, but it's sort of the ethical minimum (hence this HN post).
But I doubt many browsers, if any, work like that.