I drive a 1986 Toyota Celica Supra. It's a beautiful machine and a joy to drive, and even though it's lower in miles than yours, I've learned a few things about keeping older cars running.
You're right in that there is no specific reason not to keep an old car on the road. Even factors such as improved fuel consumption of newer cars can be offset by the energy required to build the new car in the first place and the cost of trading the car in.
It's often said that a petrol engine will need a rebuild somewhere between 150-250,000 miles. This is generally because of the piston rings, which seal the combustion chamber as the piston moves in the cylinder. They're made of very hard metal, but they do eventually wear down by scraping against the metal walls. Most cars have redundant piston rings, usually 2, perhaps 3. It's argued that having your oil analysed by specialist companies can give you a clearer picture of the engine's innards - like a blood test for cars. Failing piston rings will show as either metal shavings in the sample, or larger than expected quantities of carbon products collecting in the oil from combustion 'blowing by' the rings. An easier way to detect this is to note the consistency of the oil as you change it according to schedule. If it becomes blacker and thicker on subsequent oil changes, you're most likely suffering piston ring wear. For the most part, changing piston rings involves removing the engine - this isn't as drastic as it sounds, and a well equipped garage can probably manage it in a day or two.
It's also important to note that short journeys can greatly accelerate engine wear, as the parts are designed to align properly only when the engine is at operating temperature, and until this is reached the parts may be out of alignment.
Other components wear over time, especially oil seals. A common failure is valve stem seals, which begin to leak oil either directly into the combustion chamber (intake side) or into the exhaust stream. This can produce impressive amounts of smoke from the exhaust. It also produces a characteristic puff of blue smoke from a cold start. These also require specialist tools to change (disarming springs etc.).
The good thing about major overhauls like this, however, is that they tend to last the same length of time again. If you have no foreseeable intention to change cars, this can be a good use of money versus buying another. Once these tasks are done, they won't need repeating for many years.
Gearbox failures are pretty rare, but gearboxes do wear through use. If it's a stick-shift (manual), the gears have brass rings known as synchros, which are used to bring the gears up to speed as you shift. Brass is a soft metal, deliberately used because it slightly deforms as the gears are swapped to give a smooth shift, but over time the rings wear down. This is where you get crunching between gears as the destination gear hasn't spun up to the correct speed. It can also make shifts very jerky, which in turn puts more stress on the cogs and can lead to mechanical failure of the gears. Changing the gearbox oil in line with the manufacturer's suggestions helps a lot - gearbox oil contains many additives to slow wear of these rings, but the tight tolerances of the gears causes 'shearing' where the oil is compressed between the teeth of the cogs, which breaks down some of the additives. Gearbox oil is more resistant than engine oil, but not immune. Some gearbox designs, Subaru included, incorporate filters that should be changed together with the fluid.
Automatic transmission fluid is slightly different as it's used as a hydraulic fluid, and as such is usually under pressure, since it's ultimately what causes the gears to shift. Mostly due to this, it tends to have a shorter life than manual transmission fluid. Automatic transmissions, however, don't suffer from clutch wear due to the fluid coupling. Almost all working surfaces in an auto-box are liberally coated with oil when in use, and if the oil level drops, the gearbox will quickly stop working properly before any major damage can be done. Old fluid can cause hydraulic pathways to clog up, causing jerky or missed shifts, but a professional drain, flush and replacement can bring it back to previous condition.
Gearboxes can be rebuilt, so the wear is not fatal, but it's an expensive job. Other components that incorporate gears, such as the rear differential, are vulnerable to the same failures. However, as you've reached 200,000 miles already without problems, it illustrates how tough these components are.
The mechanical aspects are, therefore, reasonably predictable. Good maintenance can see a car reach 500,000+ miles on the odometer. Other factors can cause problems, of course. Rust, as noted, is a major problem in older cars. My Supra is currently in a bodyshop having its sills rebuilt - despite little visible rust on the outer edge, the inner pieces of metal have disintegrated. Japanese cars have a well-earned reputation for rust problems - I also own a 2003 Subaru Outback, and discovered rust in the rear driver's wheel arch (fuel filler side). Apparently this is a common problem across all Subarus and is a spot to keep an eye on - I was able to poke a quarter-sized hole through the metal with just my thumb!
Rust and corrosion can also cause electrical issues, either through causing cables to chafe or terminals to corrode over. Car electrical problems are a dark art and very difficult to diagnose as an amateur mechanic - spoken from experience here!! Japanese cars restore their reputation here by having extremely reliable electricals and electronics - my Supra's electronics still work 100% after 31 years. However, if they do ever go wrong with a non-obvious problem, you'll generally need to enlist an automotive electrician. Especially older cars where individual cables drive each device - it amounts to miles of tightly bundled cables running the whole length of the car and can fail in odd places.
Mostly, it comes down to economics, but mechanically, there are few reasons why you can't keep a car running as long as you like.
