Maybe I'm blind eyed to miss the point - I would agree that passive serial is an option for systems with e.g. an µC that configures the FPGA on power up to eliminate the requirement for a separate configuration ROM. For the MAX10 the configuration Flash is built-in, thus the MAX10 configures without an external ROM. This internal ROM needs to be programmed once in production, e.g. at the same time the board assembly is tested by using the JTAG I/F already...
What would be the benefit to program the internal ROM by passive serial I/F rather JTAG?