People are afraid of change, and with good reason

Take something you love, enjoy or like a lot, and then someone comes along and says that they want to change it or rework it. Your gut reaction would be "But I like it how it is, what if they make it worse or less fun?"

In some cases, like with DRK bloodweapon and Living dead changes, both the players and the Devs agreed they needed changes, so people were looking fowards to this changes. But in the case of PLD, most dedicated PLD players will say that they enjoy the job, and/or are afraid of another incident like the SMN rework (wich numerically succeeded by making the job more accesible, but ended up making old SMN players feel abandoned/like their job was gutted)

So, to circle around to your main question. People are anxious about the PLD rework because reworks tend to be hit or miss, it could be minimal changes that somehow make PLD work perfectly in the 2 minute meta, or it could be a fundamental change to the rotation that shifts the playstyle to something old PLD mains don't like, and that is what has people anxious