ALMA outlook and systematics. Left: ALMA flux densities in various bands for an evolving f_gas as (1 + z)^5.0 at 0.0 < z < 1.0 and fixed f_gas = 8%, at z > 1.0. The predicted fluxes are derived using the average DL07 SED presented in Fig. 6(right) and are corrected for CMB effects. The blue shaded region depicts the range of predicted flux densities in ALMA Band 3 assuming a 50% uncertainty in the adopted f_gas. Middle: ratio of M_gas estimates, as inferred for a fixed monochromatic luminosity at a given rest-frame wavelength from (1) scaling the average SED of QGs presented in Fig. 6(right), , and (2) the prescription of Scoville et al. (2017), . Similarly, the right y-axis depicts the ratio of the monochromatic luminosities predicted by the two methods for a fixed M_gas. Right: the effect of the adopted M_gas-method on the recovered evolution of f_gas. Filled blue circles correspond to the values presented in Fig. 4 using the full SED analysis (). Filled grey squares correspond to the values derived from the Scoville et al. (2017) prescription, assuming monochromatic observations at λ_obs = 850 μm, (). The empty points (and the colour-shaded region) are an extrapolation to higher redshifts under the assumption of fixed = 8% at z > 2.0.