With KMail there was never, ever, a need to enable the HTML view by default (there’s a warning in the settings window tab which should never be ignored) – this is also true for Kontact 5.3.0 with the QtWebEngine.
IMHO KMail was, and still is, a secure, nice, e-Mail client.
No. That was changed somewhere around 2010-2011 for security reasons. Mind, I never had to reapply the setting. I’ll check what the default these days is.
That depends of course on YOUR requirements. Security conscious people never really trust HTML e-mails. They think that HTML is NOT a proper format for e-mails. As many Linux users are very security conscious the default in Kmail always has been not to show HTML mails as such by default.
Maybe this helps you to develop your policy in this respect and then to decide what your preferred option is.