If the brake system were really closed then you wouldn't have to flush it ever.
You don't have to flush your brake system because you replace the pads or rotors. It's conventient to do so when you bleed the brakes which you should probably do after any brake work.
If you get a pressure bleeder then flushing isn't more work than bleeding. Plus you eliminate the risk of accelerating wear on the master cylinder seals that comes from bottoming the pedal while bleeding. Or use a suction bleeder if you don't have ASR.
Maybe you can get the same security by not pressing the pedal all the way to the floor.