I'm considering making a replacement solution for an automotive fan output stage. The original part gets quite warm and tends to be unreliable - it's cooled with massive fins and sits directly in the air stream of the fan it controls to keep it cold.
I found modern TO-220 MOSFETs with suitable voltage ratings with Rds(on) values of less than 2 mOhm, which should result in much less heat and much simpler cooling than the original part - it'll have to switch up to 40 A at 14 V.
My question: How good have MOSFET developments been in the last 2-3 decades with regard to on-state resistance? Were sufficiently low-resistance MOSFETs just not available back then such that an extensive cooling solution was required?