I don't think the coaching staff is wrong, one of the problems with developing young pitchers is they will get hurt trying to add to their repertoire.
Notice Nola was heavy four seamers early and has worked his two seamer back in, this strategy is probably a response to "analytics," beat the launch angle with high 4 seamers and breaking stuff low and away. Establishing a 95 MPH 4 seamer has made him more effective, the same way his changeup has made it harder to sit on his curve.
Establishing the 4 seamer high in the zone allows you to use the 2 seamer more effectively because they can't sit on it and "launch angle" HRs. In the same way you throw FBs inside to keep a player from diving over the late to hit your slider away.
The key to success for most pitchers isn't "stuff," but the ability to add to your toolkit and execute to keep hitters off balance, today, practically every ML hitter can pound a 95 MPH FB in the sweet zone - yet we've seen good hitting teams flail away trying to hit Vargas, he of the 86 MPH FB.
Eflin throwing both a 95 MPH 4 seamer at the top of the zone, and a 92-94 2 seamer in on the hands or breaking over the outside corner is a better pitcher than the one that can only throw one of these pitches.
The cutter is the alternative to the two seamer, but same idea, higher velocity 4 seamer with less movement high in the zone, then the cutter a few miles slower with late movement set up off the FB.
