I agree. Perhaps this is why they may simultaneously be trying to force use of "reading pane' as the preferred method. I HATE reading pane, huge security risk and hogs viewing space. Likely message covers reading pane also, I disabled reading pane (I think). Why don't they offer options for users who are paying for this product, which is currently unusable IMO.
I can't imagine anyone wanting to open one new message at a time AND have solo message hijack/block reading pane from view... blocks entire outlook window. Highly ineffective, time wasting and beyond frustrating, why not provide option to toggle on & off rather than jamming down our throats and blocking productivity and increasing hair pulling and smashing keyboards, maybe that last part only applies to me :) I still despise the huge ugly, juvenile oversized ribbon and icons hogging up screen space, catering to millennials perhaps? What about us professionals here, who prefer lean and mean, tried and true features and functionality, slowly being stripped away with each iteration.
I too have returned to prior version of outlook. Disgusted that we must beg for primary features in a "community forum" where most users will just suffer. MS should consider for each of us who take time to post feedback there are millions who agree but don't know how, or bother as they may feel stuck and overwhelmed.