There is a lovely story (probably apocryphal*) of a student taking a design proposal to the head of Cambridge University Engineering Department. The Prof looked at the plans and said
"Nice idea, Whittle, but it will never work."
The student was of course Sir Frank Whittle and the design was for the jet engine.
Whether or not exchange really happened, there is a whole cadre of such eminent thinkers, either retired or in the twilight of their careers, who regularly try to throw similar sticks into the spokes of green/low carbon technology. Letters regularly appear in the press from these chaps, typically saying:
"Before everyone rushes to embrace wind power/the hydrogen economy/electric vehicles/biomass (delete as appropriate), a few simple sums show that to replace all electricity/gasoline vehicles/domestic heating systems would require [something impossible/very expensive]. This headlong rush to do [X] is foolhardy if not downright dangerous".
Those simple sums usually assume that the technology involved is intended to replace its conventional equivalent entirely, without any change to usage patterns, without any evolution in the technology concerned and at current prices. They ignore the immutable laws of technological development - as technologies mature their costs plummet, efficiencies improve, synergies emerge and user behaviour changes to suit. But you have to start at the beginning of that cycle, you can't just parachute into the maturity phase.
The annoying thing for me is that these would-be Cassandras know this better than anyone. I don't know if they're just stuck in their ways, need to feel important and relevant, or whether they just resent the world passing them by. But given all their knowledge and experience, the world would be a better place if they would open their minds and become part of the solution, not part of the problem.
* The Cambridge angle to the story doesn't seem to match up with Whittle's biog, but he did apparently meet such resistance in the RAF.