Can I ask where you you see this demand, is it industry specific?
I was under the impression that the benefits (easy deployment to a known runtime aka the browser, open ports (80/443) in the firewall, large developer pool, potentially cross-platform use) outweigh the UI shortcomings and budgets were shifted to web-based productivity software.
End users who care about productivity want rich GUI's. Rich GUI's use screen real-estate better than "progressive" web apps and have abilities like right-click, rollover text, etc.; and it's cheaper to develop because they (optionally) can use WYWIWYG for many parts of it. WYWIWYG is just plain cheaper and easier to get right. You don't need "flow layout" specialists who can tame sucky CSS/DOM, which makes alchemy look logical.
Remember that businesses don't really care how software works as long as it does the job and is affordable in creation and maintenance. The bloated web stacks are not. It's too much code to do the same thing UI-fit tools did with less because they were closer to the CRUD/GUI domain itself. Web stacks are too fucking layered such that you spend most of the time managing communication between layers instead of on real biz logic. I stand by this and if you look at the code you should agree.
We DEVOLVED. Current web standards may be fine for social media, but not real work. We let the "cool stuff" pollute real work in cubicle-land. Oh, and git off my lawn! ;-)
I remember when ExtJS was positioned in the late 2000s to mimik rich desktop UIs and I think there are still lots of component based frameworks to use and pay for, if Bootstrap doesn't cut it. The prices are steep though.
Oracle Forms kind of worked like this (mostly before Java era), although its GUI control language was proprietary and not XML. Oracle Forms had its warts, but was a good proof concept for the GUI browser. Developers got a lot of work done under it.
I don't know why it died, WYSIWYG and GUI-friendly IDE's allowed higher productivity in that "one-man-bands" (small staff) could do the entire project. Now development is often layered into specialists. Things like UI design and ORM's have longer learning curves, which emphasizes specialists. But staff is larger per feature: less productive. (There are many times I wanted to shoot Bootstrap in the head when I couldn't get it to act how I wanted. Something in WYSIWYG that would take 20 seconds becomes a 2 day trial-and-error session in Bootstrap.)
WYSIWYG/GUI-IDE saved money. Most businesses don't run their internal productivity software on mobile anyhow. "Progressive" UI's are a money sink in practice.
The trend predictors got it WRONG: desktops live on in biz. Our anti-state-GUI web standards don't help. Businesses really want cheap rich desktop-like GUI's for productivity software.
I'm not against flow-based layouts, but they should be an option, along with WYSIWYG.