IIRC, I've seen somewhere in the wsm a statement to the effect that all brake and clutch rubbers should be replaced at max 4-year intervals - maybe they've designed in a 4-year self-destruct mechanism in the rubbers....
But as noted above, if the seals inside the cylinder go, you end up pushing fluid past them rather than pushing the clutch via the seals and hydraulics, which of course results in no clutch engagement. You can engage the gears when the engine is off because the gearbox isn't spinning.

