An altcoin defeats Bitcoin by developing superior technology, larger network effect, and stronger narratives.
BTCers are tricked into adopting a malicious hard fork (or
soft-hardfork). As a result, the BTC project mostly fails.
BTC script becomes too expressive, making it difficult to
exclude some message-types from the blockchain.
BTCers become overconfident and complacent, leading to a
culture of stasis and eventual destruction by Black Swan event.