This is real world example of real product.

Description

1. relatively small custom logic handling some custom protocol
2. some small slow management: SD Card initialization, reading a file..

This could be done as: small CPLD as glue that helps with the "custom protocol" + some MCU - the classical solution.

If we add the prices of CPLD + Flash MCU, we end up at say 3 USD at least.

Small FPGA's are also in that price range - so in this example a small soft-core processor was fit into the FPGA along with the custom logic. The soft-core was very simple and had very small code memory, so it required a lot of optimization on the software side. But as it was targeting volume production this optimization did make sense.

This was many years ago, there are new FPGA's families out now, and you can get more for the same price now.

This is one example where "MCU was eliminated" and where it also made sense - the price, that FPGA did also cost about 3 USD and it replaced BOTH the CPLD and MCU together.

Simple replacement of MCU in project where MCU can handle it all usually does not make sense.