What does this all-or-nothing mentality of solar energy detractors stem from?
Cost. According to the article in the OP subsidies for solar power in Germany account for just under 10% of consumer electric bills (note that's just the subsidies, not the full cost of solar), yet solar power only provides around 4% of their electricity. If it's not cost effective compared to the alternatives, then nothing may well be the best choice, regardless of what proportion of power it could potentially provide.
Of course, that "if" is exactly where most of the argument currently lies. It may well simply be that there is no alternative that can be as cheap as fossil fuels, and electricity costs are going to have to rise. It could be that the 10% increase is actually the best deal we can manage and solar is the best choice for providing as much as possible. But it could also be that solar really is more expensive than other power sources, and that building any solar capacity means wasting money that could provide more benefit elsewhere. If that turns out to be the case, then arguing in favour of nothing is actually the sensible choice.