That depends what you mean by mainstream. These days the cloud data centre is a mainstream market for hardware vendors. In that environment FPGAs can make sense. They might have advantages in throughput, latency, power consumption, security (no spectre), reliability (no software updates). For example, Microsoft use them for Azure virtual networking, machine learning and something inside Bing. You can imagine a world where every blade server has an FPGA. In fact, you can imagine a world where many blade servers have an FPGA and only a small support CPU.