Since we are again at the topic of changing the readiness system, I'll throw in my 2 cents.
My favourite thing about readiness is, how it forces you to rest your soldiers between missions, like in Long War. In vanilla X-Com it has always bothered me a bit, how you could just throw the same small group of people into battle non-stop, 7 days a week and more, with no regard for their well-being beyond getting wounded.
If it was up to me, this aspect could be even more pronounced.
Limiting readiness loss to missions with "supernatural" enemies would also affect the need to rest your troops, right? That would be bad.
And really, if you are being loose with the definition, most enemy groups could be called "supernatural". All of the wild creatures and zombies fit the bill, don't they? The various cults also do to some degree. I mean you are fighting creepy fish-people, russians on alien steroids, and freaking ninjas. They are all weirdos except for Exalt and Osiron.
I would like to propose, that readiness depletion simply be slowed (how much? dunno, half maybe?), to accomodate slower players and that readiness regeneration in the geoscape is then also slowed (by the same factor or even more).