100% agreed to this.
My first TW usage was 2009 with TW Classic, and the doubleclick-to-edit made for a very low barrier (at the time) to move between reading/editing.
In the last few years, my note taking has mainly been in tomboy/gnote, which does as you describe - provide a richly formatted visual output in the window which is editable at any time. There is no distinction between a reading and editing mode.
Returning to TW in the last year and picking up TW5, the loss of doubleclick-to-edit I was originally disappointed with, though there is a plugin which implements it and I’ve only set it on one of the several TW I have. The problem being that sometimes I want to doubleclick-to-select, so adds frustration in another direction. (“doubleclick = edit only if no text is selected” is an idea I’ve thought of, but I need to learn JS coding before seeing if I can tweak the existing for that)
Anyway, for my taste, tomboy/gnote style “the viewer IS the editor at all times” distinction would be wonderful - and keep a dedicated “edit the markup directly” mode for the power user gurus who want to get their hands that little bit dirtier!
(I do think an always-an-editor mode needs a more powerful undo/view-old-revisions history though, since the chance of accidentally “I typed in the wrong window and didn’t realise it till later” is much higher. I use TW via node so I get tiddlers as individual files, and shovel the whole thing into local git, but a more integrated solution would be great (and indeed, some of David Bovill’s demonstration (on the "Hitchhikers TiddlyWiki All-Nighter) of running local shell code via TW feels like a potential great step in that direction)