It’s not easy to explain something that is so completely obvious to myself.
Maybe one important point is that I use DEVONthink less for storing content of others rather than creating my own content. I.e. the vast majority of documents inside of most of my databases are .rtfd etc. docs I’ve written myself. Think of the third column with the group hierarchy as a hierarchical table of contents. While writing, I need to be able to be constantly looking at this table of contents to keep myself aware of where I am in the greater scheme of things; it’s just the way my brain works.
I disagree strongly. From my POV, the most basic difference between good and bad software (apart from trivial things such as bugs) is that good software offers you options to adapt it to your way of working, whereas bad software forces you to adapt your way of working to the structure of the software.
Of course, other people and software vendors in particular can entertain another POV. That’s fine with me; then I simply do not buy such software. What is not fine with me is if a software offers a specific, basic and defining feature at first and then removes it in an upgrade.
The three pane view is such a basic and defining feature in my POV. When I was looking initially for an information managing software, there were three points which I could not live without: a fully fledged rtfd text editor completely embedded in macOS (i.e. basically a TextEdit sibling), separated databases stored just like different documents in different parts of the fie system and the three pane view.
DEVONthink fulfilled all of this (although the database separation wasn’t as strict as I would have liked), so I bought it and hoped I could stick with it for all of my work. Now, after investing countless hours in working with this software, I feel as if the rug is pulled out from under my feet. I have a hard time finding such action acceptable.
Also, I don’t buy the argument that most users were confused by the three pane view. One of the most basic applications in macOS, and probably the first database-like application newbies encounter, is macOS Mail. And Mail has a three pane view. This has to be one of the most familiar GUI concepts for any Mac user. DEVONthink overwhelms many users with the myriad of options it offers and sometimes its GUI is inconsistent and confusing (case in point: the indecisiveness between a document based application (= multiple databases wrapped as different documents and opened in separate windows when launching them from the Finder) and a non-document based one (= other databases also listed in the window opened from the Finder with one specific database, no document proxy icon in DEVONthink 3)), but this basic option and familiar view is removed? 
No. I spent many months looking for a software that offers me what I need (including the three pane view). I will not rewire my brain because some developer felt like removing it.