There are loads of members with over 100k miles in this forum and a few with more than 200k miles. I reckon the only reason we don't have more nigh mileage members is because their cars are traded at dealers or sold to people who are not forum members ... so their cars simply vanish from the forum, but go on to live a long and happy life.
Your car is at the point where a large repair can equal or come very close to the monetary value of the entire car (depending on appearance, leather condition, dash, etc).
If you keep up with the oil / filter changes (3,500mi) and keep the maintenance (especially fluids) up to date, you have removed all of the major causes for failure.
All but one: that tricky, sneaky bastard known as "fate" or "chance". There will come a day when that crooked, wrinkled finger from on high will curve downward, point at your car and growl: "Ah HAAAA! I've been looking for you !".
At that point, you need to calmly consider your options. You can take the money you have been saving for 250k miles and use it to replace the gear box, radiator, water pump, etc. OR You can trade the car (getting near enough to nothing for it) and buy a newer car. Consider that newer car may come with newer, less reliable, more complicated equipment along with a new monthly expense called a car payment. When face with this choice, examine your present financial shape and consider the expenses you expect to have in the coming 5 years or so (a home, a new baby, etc) and decide if an additional car payment will chuck the monthly budget into the crapper.
There is no right or wrong answer here. There is just what's 'right' for you.