Luke, I think you're right. These are more accurately called "HTML-lite" emails, since they surely include tracking pixels if nothing else. But, yes, the goal is (1) quicker load time and (2) more casual or urgent feel.
I personally think marketers should be looking to use these very selectively. HTML emails with plenty of images are the norm in many verticals--and for good reason. HTML emails perform better. But there are exceptions. B2B brands have some of the more compelling use cases. And triggered emails are one place where "HTML-lite" emails can make sense.
There are lots of issues to consider here:
First, I do think you want to avoid having Gmail clip your newsletter. That creates a nice upper limit for how long your email should be.
Second, whether you include everything in your newsletter partly depends on what you've promised subscribers. But even if you've told them that your newsletter will include everything, your newsletter should help your subscribers prioritize their time. All content isn't equally valuable, so create a clear content hierarchy. Your featured story gets the hero treatment; others get smaller images and summaries; and less important stories may only be text headlines or summaries, perhaps in a bulleted list.
And third, your subscribers will let you know if you're including too much content. Look at the click patterns in your emails. Are subscribers clicking on the stories near the bottom? If not, then maybe you're including too much. A couple of tricks to use here include placing high-value content (especially an ongoing feature) at the bottom of your email to spur folks to scroll; and be sure to use your subject line and preview text to tell people about stories that are deeper in your newsletter.
Hope that helps.
Accessibility and images are not incompatible. Images are incredibly powerful tools. Don't avoid using them if they make sense for your brand and message. Instead, use accessibility best practices to ensure that you're delivering a coherent message to all of your subscribers.