If we are aiming to create a common international system of representing the fractional part of a decimal number, it makes sense to start with the system that is already in use by the most people.

No, it does not. The Countries individual choice has more weight than their number of citizens. It's probably also more work to change laws and regulations, than letting people adapt to the changes.

This is basically the classical problem of democratic systems, when they need to balance the interests of different sized groups. Numbers alone don't make a fair solution. And unless you have the power to force them, you will not convince everyone to follow you just by arguing with numbers anyway.

You are the one who proposed that the majority should not follow the minority, but now you are also saying that the minority should not follow the majority either. If your point is that nobody should follow anybody because freedom is more important than standardization, that's fine, but then it would have been clearer to just say that in the first place.

In my opinion, when we are talking about a data interchange format like CSV, having a simple, common format would be far more practical and efficient than allowing each country to decide for itself its own standard. Having dealt with exactly this problem in a global SaaS product where a minority of clients submitted CSV files with commas for decimal separators, I can say it would have made the parsing code a lot simpler and more robust if our system (and countless others like it) did not need to build in exceptions for this minority use case.