Language does change over time and trying to ignore that after it has happened can make you a pedant. However, words still have specific meanings, even if they

Language does change over time and trying to ignore that after it has happened can make you a pedant. However, words still have specific meanings, even if they can change. If you decide that "isn't" has a totally different meaning than the established one and start using that new meaning without explaining it to anyone, you'll just confuse people and fail to actually communicate. If you have a group of people that all use that new meaning of "isn't" then you can use it among yourselves without clarification but should still be aware of the potential for confusing others. Generally, when people use words they are trying to communicate something so they try to use meanings that their audience will understand. If you can clarify an incorrect use politely and without condescension, it is often appreciated and helpful.