What is the point of all the altcoins?

I'm still trying to fully understand this whole thing, and I started from a book called The Bitcoin Standard.

The book makes a really good point about the fact that it doesn't make sense for more than one single cryptocurrency to exist.

As a matter of fact, crypto exists to solve many of our current monetary issues. And one of these issues is all the different currencies in different countries and trying to keep up with their relative price fluctuations and their inherent inefficiencies.

If that's the case, then what is the point of more than one crypto? Wouldn't we just be swapping a host of national currencies to a host of crypto currencies?

Additionally, aren't all these altcoins developed around a small group of developers who usually have quite a lot of control on how the code of the currency works? Isn't this a sort of currency centralization?

I understand the appeal of investing in these altcoins for speculation.
Hell, I do it myself. I understand there's an opportunity to make a quick buck investing in these altcoins, and I often do this. However, am I correct in assuming that when cryptocurrencies become mainstream and fulfil the purpose they were created to fulfil inevitably every crypto that is not Bitcoin will basically disappear?