Well just as with softwareSerial, there are software implementation of I2C and SPI and none are designed to operate as well or replace their hardware assisted versions.
Comparing software serial performance against hardware serial performance is a bridge too far in my book
This is why I abandoned the 328 chip in favor of the 1284 chip for most of my projects anymore - as the latter has 2 h.w. UARTs.
Software I2C or SPI are not really much of an issue, since the h.w. peripherals are built from the get-go to use unidirectional dataflow, master-slave operation, and are also designed from the get-go to use multiple nodes, up to 256 or so. Whereas in general, RS232 is dedicated between 2 nodes, and does none of the other.
OTOH, on one of my robots, I wanted to try having multiple "slave" boards on one RS232 channel, so I cooked up a master-slave addressing scheme, and wired all of the slave Tx pins together using a wired-"AND" diode ckt, and it works perfectly fine. [note - NOT the wired-OR ckt].