I’m sorry that you can not follow my reasoning 
So I’m trying to explain it in a different way:
Companies are increasingly using several CMS. Usually one of them is the central CMS (for example, WP). In addition, another CMS is used to solve special things more easily than would be possible with WP. For example, this could be to build a very fast website, using either flatfile CMS or static website builder. The reasons for this are simple, it’s about the money and the amount of work (which is ultimately the same thing).
So there is obviously a need to combine the functionality of different CMS. The following articles are about a pachwork of several CMS, but each CMS works independently for itself. Of course, it is clear that a company also needs more knowledge and more specialists for this, which in turn causes costs. Therefore, the logical next step will be to demand a real MULTI-CMS, which can offer the functionality of different CMS in one go. If a CMS has the options to work as a flat file or as a database CMS and also as a static website builder, then that would be a big advantage. And if the CMS can also use themes / plugins from other CMS, which are docked as “modules” and operated via an interface function (without changing the original module), then that’s even better! The big advantage of such a MULTI-CMS with integrated theme builder and plugin builder is that the content has only to be created once. I create a page or an article in the MULTI-CMS and choose the way I want to publish it, i.e. as a webpage in my database CMS, or as a webpage in my flat file CMS, or as a static webpage, or all three together. Of course, all this is done by the MULTI-CMS, since it contains all three functionalities.
I hope you now understand a little better what I’m talking about and why this also has to do with the host servers and their capacity, because it’s always about money, at least almost always. And companies want to make money, not lose, or they can no longer pay the wages of employees and you become unemployed.
https://www.forrester.com/blogs/why-a-multi-cms-approach-may-be-right-for-your-brand/
https://torquemag.io/2017/12/multiple-cms-wordpress/
One should try to always be one step ahead of the customer’s wishes and to develop a MULTI-CMS already now, because it will not be easy!
