To be fair, people also had to be taught how to point with a mouse.
The double-click also wasn't essential, you could perform all actions using the one-click menus. The double-click was introduced as a shortcut. From the Apple Lisa Owner's Guide:
The File/Print menu contains all of the commands you need for creating, opening, closing, and storing your documents. Because you use these commands so frequently, the Office System includes a simple shortcut for performing these tasks: clicking the mouse button twice.
To tear off a sheet of stationery, click twice rapidly on the stationery
pad icon.
To open an icon into a window, click twice rapidly on the icon.
To close an open window, click twice rapidly on the window's title bar icon.
Clicking twice to close a window can either set aside the object or save and put away the object, depending on where the object's shadows are. If there is a shadow on the desktop, clicking twice causes the object to be set aside. If the only shadow is in a folder or on a disk, clicking twice summons a dialog box, which asks you whether you want the object set aside or put away.
Pointing with the mouse is discoverable though. If you start moving it - which odds are you will do sometime (even by accident) you will see the pointer moving and eventually figure it out.
You are very unlikely to discover the double click by accident.
I think it’s a spectrum. Unless the double-click delay is ultra short, I’m pretty sure you would discover it sooner or later. My point was that you couldn’t expect someone to learn how to use a computer without any training or instructions, so if you need that anyway, you can also include less discoverable features in it.
There is a trade-off between feature sets that are useful if you know them and super discoverable feature sets, in the sense that the reason some feature is more efficient can also make it less discoverable.
The original Macintosh came with interactive tutorial software demonstrating the use of the mouse and giving you practice manipulating it. The double-click was part of this instruction.
Less familiar users are often also confused whether something requires double-click or not, double-clicking web links for example. What's worse is that is often not immediately noticeable, leading to opening same program multiple times or breaking some submission form.
