It is countered by 500 mass in tmd, lowering its hp means im paying for a 350k e upgrade (not even counting the cost of GETTING the upgrade which is ANOTHER 350k e) that dies to a 250 mass structure built 10 minutes before the upgrade is even viable. It is already difficult enough to make billy cost effective because you very coincidentally seem to forget that to keep billy going you are paying the opportunity cost of devoting 4 t3 pgens to your ACU rather than, I dunno, 8 t3 air factories.
The game you cited is a game where half your team died, bottom air player decided it would be fun to go billy to win the game, and did damage with billy. He could have instead had 8 more air factories making air and done equivalent damage with air. He could have done literally anything. By the time the first billy was going off his team was 25% ahead in mass income. By the time the chicken push with the army was happening, their team had double your team's mass income. Game was lost, billy was completely irrelevant. Your chicken push could have killed the entire top and your team would still be behind. This is not a good game of billy being impactful at all.