As long as adding how about  MRS .  
“Most Recent Shill”
And the list goes on .

But how is one supposed to know at a specific moment in time without the benefit of time and experience?

By this logic, a favorite provider should be referred to as a PMF until one has decided to hang it out, and then can definitively coronate an ATF.  

I could think that a favorite provider of mine is an ATF…then two, three, five years down the road can meet a new provider who I then feel is an ATF.

I really don’t see the harm in how it’s used. Even in the OP post it’s clear you understand the different uses of ATF. Sometimes it’s okay for the interpretation to not be black and white. We would all have a lot less to talk about if everything was so clear. I would say this is not one worth being rattled about.

The argument over ATF has been chewed over here more times than I care to count. Personally, while I do have an ATF I think limiting yourself to only one is needlessly restrictive. This is only one of the interminable arguments here over terminology (like what GFE means) and they are boring.

If so, then "rarely changes" is mistaken; it can never change.  
If not, we follow the usage in sports and politics and weather where All-Time is understood to be all PAST time, and so future events can change the All-Time record or whatever.  
.
For me it only makes sense as all past time, so I currently have an ATF and a CF = Current Favorite who is still active/available though my ATF is not.

By the way, since CF = Current Favorite(s) has been in general use for years, why do you propose a new made-up acronym that is based on a prolixity, longer, less memorable, and thus redundant?

RandomFakeName,
  I get exactly what you are saying, but what if One of the providers has the qualities of both ATF & PMF?!! Then what?!