While RFFS are doing quite well at providing First Responder-level first-aid care at major airports, and have had a couple of wins with subsequent early defibrillation of cardiac arrests, there is little / no evidence that having an on-field fire response makes any difference to outcomes in GA incidents at GA airports. It was this lack of outcomes that led to their removal from GAAPs.
The response time to the site is never as quick as the speed of the fuel-fed fire. The fatalities in crash-&-burn incidents occur either on impact or within a minute or so. Look at recent history - Jandakot, the 404 EFATO crash, Bankstown, the night training single engine go-around that went wrong, the Bathurst Is C310, the Cessnock R44, the Bathurst Chieftain, the Bankstown Mojave, the Launceston Chieftain, the Mareeba Aztec, the Toowoomba Kingair.... all crashes with fires where it was all over so fast that the presence or absence of an on-field RFFS was irrelevant.