It’s a bit of a myth to consider the market value of a car when deciding if it’s worth it.
Market value is determined by age and whim – the viability of a car is related to the actual mechanical condition. Say you have a decent old £500 car where the gearbox goes, you might need £500 to fix that. But if you know the car well, it might be the only issue. If you get rid of it and get another, you’ll be £500 worse off AND then you might have another gearbox failure, or worse.
Market value is not a true estimation of anything’s actual daily worth to you. My Orange 5 for example is 5 years old and was £3k’s worth when it was new. I could sell it for what, £700? But it’s still a £3k bike and rides and works like one. There’s absolutely no sense in selling it.
Likewise cars. If you know an old car’s in good condition it’s worth as much as any other reliable car, regardless of mileage or age, if you are only interested in reliability of course.