  Considering theoretically the transition between hydrodynamic and ballistic
regimes in 2D semiconductors, we show that electrons in high-mobility 2D GaAs
are by far the best system for the direct observation of collective
hydrodynamic effects even in bulk transport properties independent of
complicated transport features in narrow constrictions and small systems where
Gurzhi phenomena are typically studied experimentally. We predict a strong
hydrodynamics-induced generic violation of the Wiedeman-Franz law in bulk 2D
GaAs systems for mobilities as modest as $10^6 \mathrm{cm}^2/Vs$ and densities
$1$-$5\times10^{11} \mathrm{cm}^{-2}$ in the temperature range of $T=1$-$40K$.
