The Access Denied issue with Outlook for Windows has nothing necessarily to do with pst files or with Office Outlook 2016, 2021, etc.
Evidence for this is that I get Access Denied erratically on a laptop that has never had Office Outlook on it, always instead using just Outlook webmail on this computer, until I sadly downloaded Outlook for Windows. Office Outlook on my desktop is POP, which has no effects on the laptop experience, no syncing, etc.
If the point elsewhere is correct about the origin of these problems being Edge and progressive web pages, then where does that leave Firefox and other browser users? We have to launch Edge to get email correctly? Huh?
It does look like some top Microsoft engineers made claims that something could be done along the lines of New Outlook in a reasonable time, but charitably put a lot of things have not worked out as promised and hoped, creating a pretty big stink. It all feels much like Windows in the 1990's.