I don't think WordPress should have a minimum PHP version that has reached end of life 2 years ago. WordPress says users should always update to latest version for security and so forth then WP it self should also update versions. We saw what not updating dependencies regarding jQuery led to. If people know that WP will always have a minimum PHP version support that is say 3 releases behind PHP latest then everyone knows what to expect and can plan for it. The current method is to random from the outside looking in.