Fire is a damage type that can be heavily resisted by monsters in endgame, so you are correct in thinking that RR is very important for fire builds. For this reason, you could think at first that arcanist is a weak choice for fire builds, but arcanist has so many other advantages that make up for the lack of RR: great OA sources, best spirit passive in the game, racial damage, nullification. The spirit boost is especially impactful if you heavily spirit dump (usual approach in the current meta of fire endgame builds). While leveling, you might find some very resistant mobs that are going to be annoying to kill, but if you follow the guide (devotions, skills, gear), it shouldn’t be a big issue (conflagration and AAR damage are very good).
Some of the most successful endgame builds use arcanist for fire damage, so don’t worry about it. Some examples: