I also disagree. The main problem is much simpler: they cost too much for fundamental reasons. The die-size for an FPGA that would have equivalent power of an Intel CPU or Nvidia GPU would be huge and expensive.
That's simply not right. The FPGA in your NIC is much more powerful for packet processing than any CPU or GPU. Yes, the CPU is better at being a CPU- but that's just a matter of using the right tool for the job.
This is incredibly naive. There are algorithm where CPU but GPU easily. There are also algorithms where a GPU beats a CPU easily. There are also algorithms where an FPGA beats a GPU and a CPU easily. You simply can't say x is better then y in general. They all have their own domains.