i think a lot of worry comes from "some guy who said he got a letter"...in my personal experience this has always been excessive data use while roaming. I myself two years ago was using around 120Gbs per month and never heard anything about it.
Also, the store caring about root/knox/etc is one thing and often just depends on knowledge of tech at store....most dont know a thing and dont have to risk it. Since you have modified your phone, they can just deny you service and note that your phone is modified. They rarely care when theyre just replacing a phone. Sprint caring about the fact your modifying/improving/ruining your phone doesnt matter unless you want them or Samsung to service it..and really it's all Samsung. YOU took responsibility for your own support when you modified, that's what it all boils down to outside of taking it to a store for "service". imho
Also, the store caring about root/knox/etc is one thing and often just depends on knowledge of tech at store....most dont know a thing and dont have to risk it. Since you have modified your phone, they can just deny you service and note that your phone is modified. They rarely care when theyre just replacing a phone. Sprint caring about the fact your modifying/improving/ruining your phone doesnt matter unless you want them or Samsung to service it..and really it's all Samsung. YOU took responsibility for your own support when you modified, that's what it all boils down to outside of taking it to a store for "service". imho