Simple if the cost of fixing it is less than the revenues from scrapping the car, repair.

Or, if the cost of fixing it is less than the annual depreciation of its replacement minus scrap value.

If when repaired it is worth more than you paid for it.. sell it..

For most owners, this would be unlikely.

Because once you start repairing an old car with rust damage, it is like treadmill.

True, but it can be a slow and fairly cheap treadmill. Our (t)rusty Focus, now passed down to an uncarconscious son, had its first weld in March 2017 (Rear o/s sill, £100) when it was sixteen years old. Three years later it had its second reconstructive surgery to the rear n/s sill (another £100). Fingers crossed for March 2021, well after its 20th birthday next month!