> I don't think the 39m primary E-ELT is designed to do full sky surveys. AFAIK it is to do targeted observations using multiple instruments.
Correct. I wasn't able to quickly find the maximum size of the FOV available at the E-ELT, but the FOVs of the first generation instruments[0] are not large enough to efficiently survey the sky. The largest FOV instrument for the E-ELT is 6.78arcmin across, compared to 3.5deg for the Rubin Observatory[1] (i.e., 960x larger area).
The E-ELT is large (39m diameter vs 6.5m effective collecting area for Ruben). So the E-ELT has a factor of 36 more collecting area.
However, the Etendue (collecting area * instantaneous FOV) of Ruben is 26x larger than that of the E-ELT. So for surveying a large portion of the sky to a fixed depth, Ruben is ~26x more efficient than the E-ELT.
[0] https://www.eso.org/sci/facilities/eelt/instrumentation/phas...