This works quite well though, I appreciate the suggestion! Generally, when linking to an external image in Markdown (in the old composer), the image resize/captioning controls are not shown unless the image is automatically downloaded/mirrorred by Discourse. Since I have that option turned off (I need to save space badly 
), the controls are not shown anyway, so as long as the image is appropriately scaled and maintains its aspect ratio, this solution you have posted works great.
I think, from a user’s perspective, I only expect to be able to do two things with an image…one is to display it of course, but the other is to be able to provide alternative text or a tooltip for accessibility purposes (e.g. so that blind users are told more than that there is a random image on the page). The scaling and aspect ratio seems to be handled quite well already, so maybe if there was just a button that allowed you to insert an external image, without having to switch into the raw Markdown editor. I think the new composer is great now that I’ve given it more time and gotten used to how it works, but unfortunately at the moment, I have to turn it off for my particular use case anyway. As such I just leave it off, but I recognize the value of the new composer for newer users as well. I look forward to being able to enable it permanently