As an amateur pilot, it sounds like this was an unfortunate course of events, but was handled properly. For example, when the altimeter stopped working, it was the right decision to turn back to Heathrow. When the second plane was deemed not airworthy, it was the right decision to not fly it. There are many reasons why an airplane can be unsafe to fly, and it's not always possible to prevent these incidents from happening, even with good maintenance. The best that can be done is thorough preflight checks while on the ground, and an emergency landing if issues crop up while airborne, however slight.

I'll concede that American Airlines could have treated its customers with more courtesy, offering them food and drink. It's certainly possible that American could have performed better maintenance on their planes. But it's good to know that their pilots are safe, and won't risk flying over the ocean if they have a slight (realistically irrelevant, given GPS) instrument issue.