You are definitely a younger technician. OBD2 can tell you a lot, which is helpful, but I will take older gen stuff any day......period. Anything TBI or MPI with/without OBD2 will run FOREVER and be cheap to fix.
Once you get to AFM, DFM, DEF, etc it gets exponentially more involved and expensive to fix. COOL a new truck can get 20 mpg with 420 hp. I had a 1996 with 305hp that got 19-21 mpg. NOTHING has improved in the last 20 years (other than the 3.0 diesel) that has made any half ton greatly improved over the last versions.
If you took CAFE out of the equation and put newer motors to the same standards as say a 1996 with OBD2, then the new stuff would mop the floor in efficiency......but the EPA and big oil won't let that happen because it would cut into their profits and motives. The improvements are there, just not usable for the public