In a well-maintained brake system with regular fluid changes and low moisture in the fluid, the master cylinder will not corrode and pushing the pedal all the way down is not likely to ruin the seals. Most cars suffer, however, from rarely-changed fluid and internal brake system corrosion. In this instance, the seals can in fact be ruined if dirt and rust line the unused part of the master cylinder bore.

Anecdotes are worth something. I just rebuilt the brake system in my father-in-law's old Pontiac 6000 prior to giving it to a refugee family. He never flushed the fluid because his idiot mechanic claimed it was "lifetime" fluid. Both front calipers and both rear wheel cylinders were seizing. I replaced these, then started to bleed the system. I slipped while holding the steering wheel, pressing the brake pedal all the way down. That one press did it: it immediately started leaking, which it had not done before. I replaced the mc and the problem was solved. The old brake fluid was pitch black.
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See the 700/900 FAQ under 'Select Link' button on the top right.