I will answer you in two ways:

The general perception:
Cryptocurrency is the family name, while bitcoin is the Head of the family. Which means that bitcoin is a big part of the family. The other members of the family are existing because the head of the family (bitcoin)is existing. The family is so united such that is the head is sick, other members of the family will be sick with him. If the head eventually becomes richer, the wealth will reach other members of the family. But there is something unfair; if any other member of the family is sick, the head will not go into sickness with him.

My personal perception

Cryptocurrency = BTC + Altcoins
Cryptocurrency - Altcoins = BTC

But Cryptocurrency - Bitcoin = 0.