Depths (3σ) of different gas mass tracers with redshift with 1 h of on-source time for an M_⋆ = 1.2 × 10¹¹ M_⊙ galaxy (including ALMA Bands 1 and 2 or VLA; see Sect. 2). For clarity, the depth reached by the [CII] line is highlighted by the grey shaded region, i.e. higher f_mol are expected as detections in the white area above it. The black solid line shows the gas fraction evolution for a main sequence galaxy with M_⋆ = 10¹¹ M_⊙ from Liu et al. (2019). The QGs from the literature are shown as white markers. Measurements from stacked samples are shown with thick black contours. The gas fraction values were converted to a Chabrier IMF wherever necessary. All CO measurements were homogenised to α_CO = 4.4 M_⊙ (K km s⁻¹ pc²)⁻¹. All dust measurements were homogenised to a G/D ratio of 92. (*) The original measurements from Whitaker et al. (2021a) have been corrected according to the stellar profile-based extraction in Gobat et al. (2022) and by using the dust template by Magdis et al. (2021).