Huge fire rigs are not super efficient; ~5 percent of calls actually require the fire fighting component of the rig's capabilities. Well-appointed Tahoes (or other big trucks) can also be super effective in responding to lower-acuity calls and can amplify a departments ability to respond to call volume (because 2-3 people in a truck can respond instead of a crew of 6 in an engine). That said, where there often ends up being a lot of pushback is from the political side - nobody wants to be the person who takes a fire truck off the line, potentially increasing response times to the areas serviced by that station. For smaller jurisdictions, they also may not have that flexibility in the vehicles because each incremental rig is that much more important. It's a lot easier to swap to other options when your n+1 is going from 20 to 21, as opposed to a smaller town talking about 1 or 2 engines.