You can observe it for a much shorter amount of time before you notice that the decay rate is constant. You don't have to watch it for 4 billion years, just like you don't have to watch your clock every second to know when an hour is passed.Have they ever observed the behavior of matter or
energy or atoms over a period of 4 billion years? Its all based on a false premise.
The other thing about zircon is its impurities, especially uranium. The uranium-lead (U-Pb) system of dating rocks has been refined to great accuracy, and U-Pb zircon dating is now a precise tool for rocks as old as Earth itself, some 4.6 billion years. Zircon is good for this because it holds these elements tightly.