To be completely honest, Bitcoin is pretty bad for an investment with percentage growth in mind, while not completely being dead. Various altcoins have shown higher percentage growth over the last years compared to Bitcoin which has been doing well itself, but not as good as a few altcoins. Its like Bitcoin is a risk-free long term investment while altcoins are completely risky investments and that's exactly why more growth can be expected from altcoins with a good foundation.
Bitcoin is by no means a risk-free long-term investment. It has had a fabulous run-up that nothing in history can match, but that doesn't mean anything for the future. Until there is a government with a strong army behind it, BTC can dissapear quite quickly. Altcoins can dissapear even quicker.
If BTC takes the fall, it drags the whole scene down from current lofty valuations.