I agree with your point about Bitcoin's purpose but I will like to add that altcoins play a very important role in expanding what blockchain can do. While Bitcoin is all about decentralizing money, altcoins have pushed the limits by creating new technologies like smart contracts and decentralized apps. This keeps the whole crypto space growing.
If we only focus on Bitcoin, we might miss out on these innovations that we are all not yet familiar with.. Altcoins encourage competition and force Bitcoin to want to improve .The real strength of the crypto world comes from its diversity and ability to adapt, not by sticking to a particular idea.