It depends on what the concern is. when IE6 was the default browser for the vast majority of consumers, it held back web technologies and generated considerable pressure towards MSFT hosting services.

And on some Android devices it isn't even the default (e.g. Samsung Internet).

Unless you are counting Android as a Linux system, Linux isn't in common use. Ask the top 5 PC makers how many Linux setups they sell and it's going to be a pretty small number.