I'm with Tiny on this one,there's a good to strong chance it is just decades of wear and tear.
I had an o/t and much younger car imported from So-Cal with the same problem,the glass almost looked media blasted it was so bad.
It was a scary drive in the dark and rain I can tell you and that was with 'proper' windshield wipers!
For me it was a no brainer,originality wasn't high on the cards plus I fit glass...
To be honest I've not heard of Bon Ami but from what Tiny and Peter H are saying it might be worth a try,you've nothing to lose.
If that doesn't work and unless you can live with it the short and easy answer is replace the glass.
