I know that Grav already pursues such an approach, that’s why I’m interested in Grav. And the approach will continue to develop. There are also other CMS that go in this direction, WP for example. It has many plugins for static website building and also for exporting entire websites to other CMS, for example Grav. It’s just a pity that all developers then develop the same thing in parallel, instead of integrating what already exists. It would be more interesting. For example, you can also install a WP within a Joomla, I have read.
Websites can be a mixture of different technologies. Let’s say you have a website with product support, no matter what kind of products it is. There you have 3 functions that you want to fulfill:
Online support with timely response for customers with a support subscription (a hotline, but without a phone)
A forum for all users, even those without a support subscription.
A documentation on the products.
Of course, everything should look like one piece, i.e. a theme, a branch. But number 3 you can build well as a static website, because there are relatively few changes (versions management). Number 2 must be more flexible, but it is not absolutely necessary to record all requests systematically, i.e. a flat file solution. Number 1 is database-based, because you want to analyze the support requests in order to better respond to the individual customer, for example with an offer for user training, or you want to know which of your supporters provide quick and correct answers to pay them a bonus, or you want to know where your documentation is not clear enough to adapt it in the next version. That’s why you need a database solution here, for better evaluation.
You see, it makes perfect sense to combine different technologies in one website and with subdirectories this is also possible without any problem. In the home you have a WP, in the directory for the forum a flat file forum, in the directory for the documentation a static website. No problem for the php server.
But ok, I think the developers have certainly recognized the idea and will stay on the way. You could form groups, for example drupal with Grav and Nikola build a MULTI-CMS together. Or WP and automad and pelican are building a second MULTI-CMS. 3-4 should be enough!
We just let it happen, the technical problems are always solvable, it’s just about the effort and the financing, but for this you have the special customer projects, where you earn with service, even if the software itself is free.