When these paints were all the rage in the 70s, it was
acknowledged that each coat reduced the microporosity, and a single heavy
coat did the same. At what point the paint ceased to be at all microporous,

Actually all paint is "porous" i.e. will transmit water vapour, and
"microporous" doesn't mean anything. For non porous you'd have to
think of bitumen+gold leaf etc. More paint means less porous but more
water proof.

Anything made of softwood that faces south/ south west will have a limited
life, whatever you treat it with.

Not so, what about those millions of bits of Victorian and earlier
joinery which are still in good nick although softwood? This is down
to good detail design (a thing of the past) and lashings of paint.
Lead paint helped of course, being better than anything modern.

I'd carry on painting with any good quality oil based paint having
first knotted with shellac.. The one essential is to go over it
carefully several times during the first few years and remedy any
defects. After everything has dried and settled etc the paint should
last 10 years or more before needing touching up again.

cheers

Jacob