Would running PureOS in a Fairphone 2 be viable at some time in the future?

Pureism wouldn’t port their OS to the Fairphone because of their principles and depending on the license(s) they used someone else could or could not make an unofficial port legally. If they used a strong viral license then their OS can not be modified to work with non-free software.

With Ubuntu and Lineage OS you don’t have this issue, because they don’t aim to be a purely free OS.

It would rather have to be PureOS and NonPureOS, since it’s no longer pure if it works with non-free software.