We present continued radio and X-ray observations of the relativistic tidal disruption event Swift J164449.3+573451 extending to $delta t approx 2000$ d after discovery. The radio data were obtained with the VLA as part of a long-term program to monitor the energy and dynamical evolution of the relativistic jet and to characterize the parsec-scale environment around a previously dormant supermassive black hole. We combine these data with $textit{Chandra}$ X-ray observations and demonstrate that the X-ray emission following the sharp decline at $delta t approx 500$ d is due to the forward shock. Using the X-ray data, in conjunction with optical/NIR data, we constrain the synchrotron cooling frequency and the microphysical properties of the outflow for the first time. We find that the cooling frequency evolves through the optical/NIR band at $delta t approx 10 - 200$ d, corresponding to a magnetic field energy density fraction of $epsilon_B approx 10^{-3}$, well below equipartition; the X-ray data demonstrate that this deviation from equipartition holds to at least $delta t approx 2000$ d. We thus recalculate the physical properties of the jet over the lifetime of the event, no longer assuming equipartition. We find a total kinetic energy of $E_K approx 4 times 10^{51}$ erg and a transition to non-relativistic expansion on the timescale of our latest observations ($delta t approx 700$ d). The density profile is approximately $R^{-3/2}$ at $lesssim 0.3$ pc and $gtrsim 0.7$ pc, with a plateau at intermediate scales, characteristic of Bondi accretion. Based on its evolution thus far, we predict that Sw 1644+57 will be detectable at centimeter wavelengths for decades to centuries with existing and upcoming radio facilities. Similar off-axis events should be detectable to $z sim 2$, but with a slow evolution that may inhibit their recognition as transient events.
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