I can't help but suspect you're right.
Obviously photosynthesis isn't going to be efficient as an energy source, but scientists keep finding other energy sources which maintain life.
On a world with just those energy sources would life be sustainable? Seems to have been exactly that way on the early Earth.
I think there are surprises out there if we keep open minds. Just as there have been some real surprises here, in the deep oceans and under the Antarctic ice.
With energy and liquid water, DNA seems to find a way to scarf it up and protect itself with cell walls. We call it "life".
Arf