Well actually they make things about as good or better than they used to. Remember that mars helicopter that was only supposed to last a few flights?

If anything, they are building them better. Their rovers now vastly exceed their original missions. Satellites now form an interplanetary internet network. We even have a helicopter on Mars! It's reached the point where it's playing spotter for the river, since they've run out of experiments to run on it!

There was 1 (experimental) mission into using ion drives. Multiple drives failed. 1 only lost its +ve ion thruster, another only it's -ve thruster. They managed to reconfigure it in flight. The 2 drives were interconnected via an FPGA processor on board. They rerouted the engines through the computer!

Don't get me wrong, Voyager I and II are amazing. However, given the same opportunity and funding now, the result would blow them away in almost every department, including longevity.