BA policy is different from that of several other airlines, and does permit customers to consume their own alcohol on board. They will only stop it if behaviour becomes inappropriate as a result. There are remarkably few issues, particularly in comparison with certain flights on other airlines, and fear of something potentially happening (which from your story didn’t come to fruition) is not reason to change it for everyone.

As noted above, there are plenty of BA flights where customers up front consume as much as this guy did, often after doing similarly in the lounge before getting onboard, and we don’t have regular threads where they then caused problems.