I spent a lot of time thinking about this, and the conclusion I reached is while the cpu may be cheaper, it can generate revenue where the fpga cannot. So even thought the tco may seem upside down the opportunity cost of using a few cores makes up for the additional cost.

Gcp for example has the potential for ~1 k revenue per core over a system lifespan. A smart nic is probably ~1.5k, so saving 2 cores outs you in the black and has other security advantages.