I have the same worries, but I console myself this way.
I paid $15,000 for a 2000 2.7 Boxster with 56,000 miles. If it doesn't die, it will probably always be worth at least that much. If the engine dies, I spend $15,000 on a 3.8l conversion and the car will be worth just south of $30,000 (at least that's what I have seen them sell for). Either way, I can't lose money on the car. I do all my own work, so maintenance is cheap. I love this thing. I used to buy a new car every few years and flush money down the toilet. I'd have 10 Boxsters with all that wasted depreciation. Your car is a win/win situation, stop worrying.