Not the OP but have a 20-year-old car. The relevant calculation is not cost of annual repair v value of car, but rather annual cost vs annual cost of a new car. Even if you amortize the upfront cost of a new car over 20 years, the increased insurance cost and (depending on where you live) property taxes plus some annual maintenance, at least for me, is substantially more expensive than annual maintenance on my current car.

I did a from-the-ground-up rebuild (including the engine) just after buying it. That cost an arm and a leg but all in (including the original car) it still came to ~half of what a new one would cost. Anything that had been 'improved' on it was brought back to stock. It's been super reliable, I've had it since jan 2020, put a considerable number of kms on it and it hasn't let me down (so far :) ).

As for doing the maintenance myself, I don't have experience with this kind of car at all, I've worked a lot on classic Mini's, Citroens (2CV and DS) and Austin Maxi. But never anything like this so I'm more than happy to let someone else earn a buck on it. But it's been pretty cheap to run so far, fuel, oil, regular service and once a control arm that got bent out of shape.

Compared to a new vehicle I'm considerably better off.

That would not be the case amortized I expect. You can sell virtually any car for $5k as a floor price I’d say. Most yearly maintenance amounts to changing oil. Maybe tires every four years. Every 5-10 years maybe a bigger couple hundred dollar job. That has been about my experience owning used cars. But still well below $5k/yr.

> I guess at some point the yearly maintenance costs exceed the value of the car itself.

This is often mentioned but is not relevant.

In terms of cost, what matters is whether an equally good (for whatever metrics a car is "good" to you) replacement car will cost less or more.