Bitcoin is certainly unique in being the first cryptocurrency.
But other coins can have unique qualities that Bitcoin can never adopt.
It’s true that new altcoins can have a unique feature that Bitcoin can’t have because it’s already developed but other altcoin can’t copy the Bitcoin uniqueness which is the most important in cryptocurrency which is the real decentralization.
One could argue that altcoins are made specifically for this.
Altcoins are basically experiments of what other features could be improved from bitcoin or what other features could be created that are not implemented in bitcoin. Altcoins may not have yet surpassed what bitcoin has achieved but they are reaching technological milestones that bitcoin has yet to achieve.