It's very important to realize that Carbon-14 dating cannot be used to date anything older than 100,000 years, even with the most advanced technology we have today. The half-life is not long enough for traces (i.e. measurable amounts) to last that long. So it's a misnomer to think that Carbon-14 dating is responsible for dating rocks, fossils, etc. beyond that time frame.
Instead, other radiometric methods like Potassium-Argon dating are used, which have much longer half lives. Even with these methods, however, we must remember the concept that we can't "measure" age. It's not a physical property we can observe like mass or volume. The concept behind these methods is to measure current decay rate, and work back from the present proportion of decay elements to find the assumed starting point. We do have to rely on the assumption that decay rates have been concept, that we can accurately estimate the starting element, and that it's been a closed system for the entire time.
So yes, even the most "conclusive" dating methods rely on assumptions. There's no way around it. There is even good reason to reject these assumptions on occasion. For instance, Mt. Saint Helens gave us rocks with known ages because of observation, which we saw to have much larger proportions of decay elements than we typically assume, leading dating methods to date them at millions of years old when in reality they were less than 50 years old.
