Simple: because at high levels of intermittent penetration, you have to build so much "non-productive" storage technology that the Energy Return on Investment is below the level thought necessary to sustain industrial civilization. For example, this was just published last week:
(Iñigo Capellán-Pérez, Carlos de Castro, Luis Javier Miguel González,
Dynamic Energy Return on Energy Investment (EROI) and material requirements in scenarios of global transition to renewable energies, Energy Strategy Reviews, Volume 26, 2019, 100399, ISSN 2211-467X, https://doi.org/10.1016/j.esr.2019.100399.)
Which actually answers why the point you bring up is not relevant:
"In a recent meeting of scientists and economists in London, economists raised eight points as to why it was not necessary to consider EROI in determining future energy availability or policy."
Now the above paper tries to refute those points, but using invalid logic, e.g. arguing that because EROI and costs are linked in oil and gas, then the same observation must hold across categories to renewables.
The red flag that stood out to me is that they show huge increases in tellurium, gallium, and indium demand in Figure 10. Those materials are only required for thin film solar technologies. But according to Table 2, their scenario includes one PV technology: fixed-tilt arrays of silicon PV. Where are the increased demand for tellurium, gallium and indium coming from? It reads like they copy-pasted information from prior studies without paying attention to their own scenario parameters.
[1] https://www.sciencedirect.com/science/article/pii/S2211467X1...
(Iñigo Capellán-Pérez, Carlos de Castro, Luis Javier Miguel González, Dynamic Energy Return on Energy Investment (EROI) and material requirements in scenarios of global transition to renewable energies, Energy Strategy Reviews, Volume 26, 2019, 100399, ISSN 2211-467X, https://doi.org/10.1016/j.esr.2019.100399.)