Repairing a known item makes sense when the costs are small compared to a replacement. I have old bicycles that are worth nothing and I am quite happy to spend maybe £50-£150 in components to fix. Costs are much lower than buying a new good bike or trying to find another good second hand bike (which I would need to fix anyway).
However that frequently isn't the case with car. It is £300-1000 every time I need to get it repaired. This happens at an increasing frequency as the car ages. This can then add up to several 1000s quite easily.
e.g In the last year I owned that Astra I had to to fix the following issue (all costs include labour):
- New Clutch (and it needed to be towed to the garage as the clutch wouldn't engage). £1000 for clutch replacement, £150 for the tow. I also needed to rent a car to drive to a funeral. That cost me another £100-200 IIRC. So we we are up to £1350.
- Car would randomly go into Limp mode. New sensor cost £300. Vehicle was never really fixed. It just went into Limp mode much less, so there was another problem somewhere. Which would need another trip to the garage.
- Service. This flagged several issues with the car. All these small repairs was another £800.
I am already upto £2,450 and I know I've forgotten stuff. I bought the car for £4000 originally.
If it was something like a classic barn on wheels Volvo which are bullet proof, or a classic Rover like a Rover P5. I might be willing to keep dumping money into it. But it isn't, it is a Vauxhall Astra.