…while being hundreds of times bigger and also not offering the scope to implement hardware hacks (e.g. interfacing through GPIO/I2C/SPI/Serial without additions that cost more than the entire Pi).
Yes - obviously you can use an old PC for many different PoCs and prototypes. It doesn’t take much imagination to think of those you can’t.
Nearly every "old PC" does offer built-in easily accessible I2C - as "DDC channel" of a monitor plug - VGA, DVI, HDMI, DisplayPort - all have it. Linux exposes those as regular /dev/i2c-*, so any I2C software that works on a RPi or other Linux SBC, will run with those too:
Yes - obviously you can use an old PC for many different PoCs and prototypes. It doesn’t take much imagination to think of those you can’t.