I don't see the need to differentiate goals "from open play" as opposed to (presumably) penalties in Billy's case. They still need scoring, and enough are missed every week around the world.
Does that mean that free kicks don't "count" either?
Scoring 14 in the championship at 35/36 is a hell of an achievement for a player who is in the twilight of his career, by any metric. And with luck there might be 1 or 2 more if he is fit enough to take part in the last 3 (or 5 or 6).
Do we need to plan for the future? Definitely. But as long as he is fit and raring to go, there is a place for him. Not starting every game certainly, and especially not when we sometimes have to play 2 or 3 in a week, but currently we simply do not have anyone better, and finding someone as good, or better (unless Brewster comes back next season and fulfils the potential we signed him for) will be eyewateringly expensive.