The value of backwards compatibility is directly proportional to the ratio of "value of user time" / "value of dev time". Depending on whether you are the dev,

The value of backwards compatibility is directly proportional to the ratio of "value of user time" / "value of dev time". Depending on whether you are the dev, the user or a third party, you may attach different values to that ratio. In the extreme, a dev might think that ratio is 0 and a user might that it's a large number. (People arguing that ISOs are already widely supported and useful are implicitly attaching a large value to that ratio) I don't think there's an objectively correct value that can be assigned to that ratio, hence disagreements about whether Really Old Thing should be supported.