I'm willing to bet you just had a stupid sales rep, if you talk them into upgrading to GPON (Even if that means ordering 150/65 for a day just to lower it back to 75/35 after they install the new ONT) you may have to explain the thinking behind that part to them.
That's why I usually just ask for a super when they tell me something I know isn't true. Thee supers sometimes have somewhat better critical thinking skills than your regular sales drone, sometimes...
Verizon's customer service is a PITA, all the money goes to the network. Always has, always will.