It would also imply that you’re dead and floating through the void of space for all eternity. Who knew positive thinking could be so depressing?
Slightly different from what the OP is asking, but “something that requires investing efforts for unlikely return” will be:
“A wild goose chase”
I’ve heard black hole, treadmill, and bottomless pit all used to describe high effort project tasks with the same reward as low effort project tasks.
If that is indeed the context.
But the OP hasn’t really given an exact context, so everyone here is just throwing out answers blindly.
I finally found a thread that plays to my strengths!
But those imply no reward or futility more than anything.
The opposite of “easy pickin’s” is tasks that are difficult and/or the payoff is not high.
Low hanging fruit has high ROI (Return On Investment). One opposite are items with low ROI.
Other terms we use in project planning are Long Poles (things that are required but will take the longest amount of time so they set the schedule) or Heavy Lifting (things that aren’t easy so they will require resources and time, but are necessary and provide high value).