The most obvious problem with this line of reasoning is that miners make more money when they are constantly mining than when they only mine opportunistically. This has already been demonstrated in the real world, with miners generally clustering around energy that is cheap and always available. We have seen miners keep gas-burning plants that would have shut down online, bringing already shut-down coal plants back online, and clustering in countries and regions where coal/nuclear/etc. energy is cheap and abundant (China, a few former Soviet republics, etc.).
Moreover, anything that consumes power and that does not need to run round the clock would have the same effect on solar deployment. Why not create a smart washing machine that waits for energy prices to fall before starting a load? Why not a steel mill? A basic flaw in the argument is the assumption that there is no better use of energy than Bitcoin mining or that Bitcoin is unique in its "flexible" energy needs. Neither assumption is true. There are alternatives with the same "flexibility" that have a clearer benefit for humanity, like direct conversion of atmospheric CO2 and water into gasoline (still a research topic, but prototype facilities are being tested at small scales right now) or any other energy storage technology (which in theory could be operated separately from generation).
Finally, a basic economic point: energy spent on cryptocurrency mining is energy not available for anything else. Even if all Bitcoin mining was exclusively powered by solar, wind, and hydro power, Bitcoin would still be slowing the progress on phasing out fossil fuels by consuming so much energy. Electric cars need to be charged whether or not Bitcoin mining occurs, and if renewable sources are all being sunk into Bitcoin then cars will be charged using non-renewable sources.
I know you asked for the other side, but sometimes there is no other side and this is one of those times. The idea that Bitcoin is actually good for the environment because it could, possibly, subsidize solar production is just a desperate attempt to work around the criticism that Bitcoin wastes energy. Yet that criticism of Bitcoin is very easy to demonstrate: divide the number of transactions processed in one second by the power consumption of Bitcoin, and compare the same estimate for Visa or whatever. Even if the estimated power consumption of Bitcoin is off by a factor of 100 you would still be better off with Visa, since Bitcoin processes 7 transactions per second while Visa processes about 1700 (and Visa's annual reports on the topic always show much lower power consumption figures than even the most generous estimates for Bitcoin -- several orders of magnitude lower).
I'll give credit where it is due: people who reflexively shout "PoS" when these criticisms are raised are at least acknowledging that PoW is a problem that needs to be addressed. That is the only actual counterpoint to the complaints about energy consumption, a technical improvement that is ready for deployment (and is already partially used by Ethereum, with a full switch supposedly coming soon).