As far as “interesting and new application that can’t be done easier in a microcontroller/SBC” goes, I think for some things the novelty of just doing it on an FPGA can be worth the barrier to entry. Perhaps not always, but there’s a difference between, say, an Apple II emulator running on a Pi and one on an FPGA that behaves as the original hardware did, and may even be compatible, to an extent, with the original hardware.
After last night I stayed up late reading on all sorts of shenanigans, and while it pains me to think just how ill informed and naive I was less than 24 hours ago, the appeal of FPGA’s is really starting to grow on me. I’d argue the barrier to entry may not be that people cannot think of projects to do on them that would not be simpler on an SBC, but that they may not understand just how flexible FPGA’s really are.
Perhaps that flexibility may scare people, “infinite possibilities” doesn’t exactly give you the slap in the right direction that “You’ve got 32k of program space and 2k of RAM, just write as many blinking LED’s as you can until it’s full” does.
I’d say it was the lack of apparent, well, not constraints, but perhaps structure that scared me away from FPGA’s at first, but now I know it’s not quite as bad as I probably psyched myself into thinking it was.