The old adage for car engines goes… “there’s no replacement for displacement.” This holds true for speakers as well. It’s a small speaker but I think it does a great job for its size. If you’re looking for louder, punchier, more filling sound, the full size HomePods are phenomenal in that regard.
To answer your question, it’s anyone’s guess. Apple has tweaked the max volume in the past, it seems, via firmware but who knows where they’ll settle at or if we’re there now. It might be at a point they believe the highest volume can be achieved while prolonging hardware life.