Markdown is as close to an open standard as we have. Sometimes open standards worked on for years by smart people are the way they are for good reason.
Change them at your peril.
Discussion
I like Markdown and I used it in all my projects, saas ecomerce and cms, from the beginning (2004).
I'm pro to open standards, interoperability and sensible retrocompatibily.
But nothing is perfect, needs and use cases change over the time; I can see how Markdown standardized the italic / bold / bullet point syntax, that is now used in a lot of contexts even by non techical persons, but images and links never took off, probably for their syntax's complexity. Can we brainstorming about a semplification using Nostr as real use case? I think yes.
Practical reasoning:
Which are the case where someone want to write down an url image? Are they more frequent than where one wants to view the image inline? I don't think so. So make sense priorize the inline usage with the simple syntax and use an alternative one (es. \
Counter argument: the simpler syntax works only it the url is a "speaking" one, so with a standard image extension. Some cdn don't output a similar permalink and so the regexp will skip and show them raw.
Several aspects to think about!