KDE for life then?
Gnome
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What would middle click do instead?
Whatever action is bound to it in applications that use it. It's a mouse button like any other. You can scroll a page in Firefox or click-drag across a surface in certain applications. That's actually the most common usage for the middle mouse button.
Seems like that should be still configurable for either action at the OS level and certainly (at least through about:config) in Firefox’s settings. Or are both entities really going to prevent that behavior?
Applications like Firefox can register the middle mouse click, but they can't reasonably prevent the paste action. This is a much saner default and I have no idea why this is such a big deal.