I agree with @bojanz in that there are external factors in driving PHP 7.x adoption and dropping support for PHP 5.x. I realise that it is not easy for local installs and shared hosting but there are some alternatives there to make the process a bit easier. We should document those in the announcement. I think that is important because realistically, we have to drop support at some time and I think it is better we do it before PHP itself drops support.
I think that Drupal 9 discussion is just a tangent to this discussion. I seem to recollect a discussion that dropping support for PHP versions does not count as an API break, and hence is possible in Drupal 8.x cycle itself.
Having said all this, I +1 on @jhodgdon's suggestion to promote this discussion to the wider community.