The big question is can you live without kidney? I guess the answer is NO so therefore Bitcoin is the kidney of digital currencies.
It makes more sense that you can't live without a heart, while you can still live without a kidney. Then why the kidneys, not the heart?
Some people consider bitcoin to be the king of cryptocurrencies, and some people consider bitcoin to be the father of cryptocurrencies. Regardless of their assumptions and mindset about bitcoin, it is inseparable from the technology it has brought to the financial system. Bitcoin is nice decentralized money to gain more trust than the old traditional financial system, but I wouldn't be too whimsical to make similes.