Whirlibird, Rivet built AKs can also be a problem I would think when the mileage gets real high. Have you ever seen it and is it repairable? Or should the gun be junked parted out at that point.

Rivet builds actually last longer, the tightness caused by the swelling of the rivet into the receiver holes prevents the movement that is present in screw builds. By reducing movement the stretching is reduced.

Repairable, that depends on the skill of the TIG welder you know.
The cost of a new receiver, or a properly hardened flat (Important!), if you have the means and tools to punch a flat, is still spend less than an hour ‘welders charge’.

Bluntly, if you’ve got that many rounds through an AK, you could afford a new receiver or even a new gun.