the adc connector was mostly proprietary in the sense that nobody else used it. third party manufacturers had no problem making adapters and cables without apple’s permission, as evidenced by the the belkin dongle this article is talking about.
what you’re really asking for is an industry standard, which is different from an open standard. however, an industry standard requires the industry as a whole to buy in to it. if they say no, but you still want to solve a problem that their existing standards do not, what do you do?
industry standards also do not typically appear overnight. usually, companies put out multiple solutions trying to solve new problems, and eventually the industry coalesces around a preferred solution. USB was introduced in 1996 with full support for mice and keyboards, but it took nearly a decade to become the de facto connector for mice and keyboards.