The 1985 Thunderbird is the thing which changed everything in NASCAR.
I suspect that the car singularly most responsible for the change in splitters in NASCAR was the 1979 Mazda RX-7 in IMSA GTU.
Resetting splitters back to 1979 would be interesting. Speeds would fall. I also suspect that the newly less aero dependent cars would be less susceptible to producing bore fests at places like Kansas.