  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.
