I've had some success with rust treatments.
Bilt Hamber do the best stuff without a shadow of a doubt.

I done some work on a MK1 Focus about 2 years ago. Arches were far worse than yours. I cleaned the car and degreased all the inner arch and the car itself. I used a few wire brushes and flap discs on a grinder to get back to decent metal.
Treated all the bare metal with Bilt Hamber Deox Gel to ensure there was no rust left behind.
Sprayed the entire inside arch and the now ground back edges with epoxy primer. Put fiberglass behind all the holes in the arch and once set I skimmed the holes with filler. Prepped and painted the outer arch. Used Bilt Hamber Dynax UB on the inside arch.

Seen the car about 2 months ago and still looks absolutely fine.