To add to your point with a very clear example: If I did a study to check the average age of people in a country where I mainly checked the age of people living in retirement homes, the margin of error would be huge even if I got the age from hundreds of thousands of people.
In more general terms: there can be systemic errors due to methodology that no increasing of the number of samples will remove.
It doesn’t matter, a margin of error exists regardless of the data source.
To add to your point with a very clear example: If I did a study to check the average age of people in a country where I mainly checked the age of people living in retirement homes, the margin of error would be huge even if I got the age from hundreds of thousands of people.
In more general terms: there can be systemic errors due to methodology that no increasing of the number of samples will remove.