For me the question of "flexibility" ties to the tools. Anyone who's ever tried to do FPGA development will know that it's not exactly agile. Third-party improvement of tools is blocked, because the "bitstream" format is confidential. Imagine if Intel or ARM refused to give you an instruction set manual and made you use a slow proprietary compiler with encrypted outputs.

Without great improvement and opening of the tools, FPGAs aren't going to be a general-purpose accelerator. It'll be a question of building one specific piece of firmware for one specific project and then deploying it semi-permanently with minor revisions across the cloud.

Wake me up when Visual Studio has an FPGA backend.