I disagree that you can make such a sweeping statement without evidence, having worked on HyperTIES [1] [2], an early hypermedia browser and authoring system with Ben Shneiderman, who invented and published the idea of underlining links, and who has performed and published empirical studies evaluating browsing strategies, single and double clicking, touch screen tracking, and other user interaction techniques.

Hyperlinks do not necessarily have to be triggered by single clicks. In HyperTIES, single clicking on a hyperlink (either inline text or embedded graphical menus) would display a description of the link destination at the bottom of the screen, and double clicking would follow the link. That gave users an easy way to get more information on a link without losing their context and navigating away from the page they were reading. Clicking on the background would highlight all links on the page (which was convenient for discovering embedded graphical links in pictures). [3] [4]

The most recent anecdotal evidence close at hand (in the sibling and grandparent comments to yours) that it's confusing is that nailer did indeed confuse double clicking with single clicking in his memory, not remembering that he subconsciously double clicks on Macs all the time.

I would argue that much in the same way the Windows desktop gives users an option to enable single-click navigation like web browsers, web browsers should also give users an option to enable double-click link navigation like HyperTIES, so a single click can display more information and actions related to the link without taking you away from your current context, and a double click navigates the link. (Of course in the real world, scripted pages and AJAX apps probably wouldn't seamlessly support both styles of interface, but double click navigation could be built into higher level toolkits, and dynamically applied to normal links by a browser extension.)

In order to make a sweeping statement like "Following a link and opening a directory are distinct enough in most people's minds to not confuse the two" you would have to perform user testing -- you can't just make up statements like that without any supporting evidence. Can you at least refer me to some empirical studies that support your claim, please?

[1] http://www.cs.umd.edu/hcil/hyperties/

Starting in 1982, HCIL developed an early hypertext system on the IBM PC computers. Ben Shneiderman invented the idea of having the text itself be the link marker, a concept that came to be called embedded menus or illuminated links. Earlier systems used typed-in codes, numbered menus or link icons. Embedded menus were first implemented by Dan Ostroff in 1983 and then applied and tested by Larry Koved (Koved and Shneiderman, 1986). In 1984-85 the work was supported by a contract from the US Department of Interior in connection with the U.S. Holocaust Memorial Museum and Education Center. Originally called The Interactive Encyclopedia Systems (TIES), we ran into trademark conflicts and in 1986 changed the name to HyperTIES as we moved toward commercial licensing with Cognetics Corporation. We conducted approximately 20 empirical studies of many design variables which were reported at the Hypertext 1987 conference and in array of journals and books. Issues such as the use of light blue highlighting as the default color for links, the inclusion of a history stack, easy access to a BACK button, article length, and global string search were all studied empirically. We used Hyperties in the widely circulated ACM-published disk Hypertext on Hypertext which contained the full text of the 8 papers in the July 1988 Communications of the ACM.

[...]

Today, the World Wide Web uses hypertext to link tens of millions of documents together. The basic highlighted text link can be traced back to a key innovation, developed in 1983, as part of TIES (The Interactive Encyclopedia System, the research predecessor to Hyperties). The original concept was to eliminate menus by embedding highlighted link phrases directly in the text (Koved and Shneiderman, 1986). Earlier designs required typing codes, selecting from menu lists, or clicking on visually distracting markers in the text. The embedded text link idea was adopted by others and became a user interface component of the World Wide Web (Berners-Lee, 1994).

[2] http://www.donhopkins.com/home/ties/LookBackAtHyperTIES.html

Designing to facilitate browsing:
A look back at the Hyperties workstation browser

Ben Shneiderman, Catherine Plaisant, Rodrigo Botafogo, Don Hopkins, William Weiland

Human-Computer Interaction Laboratory,
A.V. Williams Bldg., University of Maryland,
College Park MD 20742, U.S.A.

[3] https://www.youtube.com/watch?v=fZi4gUjaGAM

University of Maryland Human Computer Interaction Lab HyperTIES Demo. Research performed under the direction of Ben Shneiderman. HyperTIES hypermedia browser developed by Ben Shneiderman, Bill Weiland, Catherine Plaisant and Don Hopkins. Demonstrated by Don Hopkins.

[4] https://www.youtube.com/watch?v=hhmU2B79EDU

Demo of UniPress Emacs based HyperTIES authoring tool, by Don Hopkins, at the University of Maryland Human Computer Interaction Lab.