This is for the benefit of people updating from older (3.x and 4.x) versions. If we didn't install the new plugin then they'd get stuck with the old one. The old one, however, is trying to look for a PHP file which no longer exists and causes a PHP fatal error. Since this is a system plugin it gets loaded in every page of your site. The PHP fatal error would, therefore, bring down your site.
Of course you might wonder why are we not trying to uninstall this plugin when we install the package. We actually tried that and ran several tests. We also have a valuable 10 year experience of helping people with various installation issues. This method mostly works, except for a few cases: the package installation is interrupted (timeout or server issue); Joomla "forgets" to copy some files; Joomla doesn't run the installation script; database server error; a broken code cache leads to the wrong code being executed (I kid you not, that's what happens when the defunct since 2012 eAccelerator is installed on PHP 5.4); permissions on the filesystem do not allow the plugin to be removed. In all of these cases you're left with the old plugin which brings down your site. Cumulatively they account for about 5% of installation attempts. In our opinion inadvertently bringing down 5% of all sites running our software is quite bad to say the least. The only solution to that problem is installing a dummy plugin which disables itself upon first run.
So the answer is that we have to do that to prevent the sites of people upgrading from old versions being brought down and that we have to do that because a significant amount of our users is running sites on servers which won't work properly if we merely tried to uninstall that plugin.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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