Has JWST Seen Everything? Hunting for "NIRCam-dark" Dusty Star-Forming Galaxies at $z\simeq 6-9$ with ALMA2040

JWST/NIRCam has recovered the stellar counterparts of many ALMA-detected, formerly HST-dark galaxies, yet ALMA has started identifying dusty star-forming galaxies (DSFGs) at $z>6$ that remain extremely faint or undetected even with NIRCam. These “NIRCam-dark” DSFGs may trace a missing rapid-growth phase connecting UV-luminous galaxies at $z>10$ to massive quiescent galaxies at $z\gtrsim 4$. So far, the small, heterogeneously selected NIRCam-dark sample is insufficient to determine their abundance or role in the evolution of massive galaxies. We show that a blind 1.3-mm continuum survey with ALMA2040 can provide the first high-significance census of this elusive NIRCam-dark DSFG population across $z\simeq 6-9$. We conduct sensitivity and survey-speed calculations requiring a $10\sigma$ detection of a NIRCam-dark DSFG with a dust-obscured star-formation rate of $60M_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$ at $z=8$. At a like-for-like 7.5-GHz normalization, ALMA2040 will reach the required 1.3-mm RMS of $21\mu$Jy beam${}^{-1}$ about 90 times faster than current ALMA. With 100 hr on-source, ALMA2040 can cover an area of $\sim 0.5$ deg${}^2$ and detect $⟨N⟩\sim 16$ NIRCam-dark DSFGs over $7.5<z<9$, assuming a fiducial number density $n=3\times {10}^{-6}$ cMpc${}^{-3}$. Dividing the survey among independent fields with deep JWST coverage would mitigate cosmic variance and enable immediate NIRCam, MIRI, and spectroscopic characterization of their environments. The $\ge 512$-GHz instantaneous bandwidth expected for ALMA2040 will also detect [C II] line at a wide redshift range, supplementing the continuum discovery with redshift confirmation. Such a survey will test whether NIRCam-dark DSFGs trace a missing rapid-growth phase between the earliest UV-luminous galaxies at $z>10$ and the rapid emergence of massive quiescent systems at $z\gtrsim 4$.