All this talk of archeology, has inspirede to make a klatooinian archaeologist, in the vein of Indy (but sounding like his father) that is searching for remnants of culture that the hutts didn't destroy, but sold off long ago.
He likes a good rumble in the search of his lost culture, with the tagline (performed in my very bad Sean Connnery accent) "I call THIS archeology!" As he throws a punch.

It's not about "cheating" the system, and nobody's whining.  My position is that the talent is a poor addition to the rules.  Just because it exists doesn't mean it SHOULD exist.  
 
The core mechanic of the game is a narrative dice system.  A table is provided for use as inspiration, as a guideline for the relative strength of effects that can be invoked with X amount of threat/advantage.  You stated yourself that prone is a lower-powered option compared with the other effects that 3 advantage are worth.  
 
A GM is well within their rights to allow effects of comparable strength that are not listed on the table.  It's encouraged, and an intrinsic part of the narrative system (see p. 205).  Thus, the game already provides a way for melee characters to knock opponents prone.  No GM is going to deny that to a player with a Triumph (3 advantage may be debatable).  Unless the GM is a hardcore rules-lawyer that uses the existence of the Knockdown feat to say "you can't do that," but that kind of mindset goes against the spirit of the system.  Most of the other rules advocate free-form creativity, and I'd argue that those should take precedence over those that restrict options unless you pay the "tax."
 
Going by a strict reading of RAW the only value I can see is that knocking someone prone would be subject to GM approval, whereas the Knockdown talent is guaranteed.  You can point it out to the GM and say "but this says I can do it, and I paid XP so I'm not getting cheated out of it!"  But is that "safety net" worth the XP (20 XP, in the Fringer's case)?  Especially considering that a) most GMs won't veto a suggestion of that power level, and b) ALL rules are subject to GM discretion.  
 
Let's not forget that this is a TTRPG, and that one of the strengths of the medium is having a human brain running the show.  If a rule isn't working it's ALWAYS within a GM's right to ignore it, altogether or situationally, and the crux of my argument is that this talent is a GREAT candidate for that.

Yeah, that's my big problem with this talent, is that I feel knocking an opponent prone should already fall under the purview of a Triumph.  I mean, I think disarming an opponent is worth more than knocking them down, and that only costs three Advantage...

I think that might have been my thread where that was brought up.  Oops, I hadn't verified that from the actual core rulebook.  I use a consolidated and simplified table for reference that I printed up.  My group's been running prone for 3 advantage since that's equivalent to the amount of threat that can trigger it.  Which, given that the table is meant to be a guideline (and prone can fall under the umbrella of "narrative advantage"), isn't so unreasonable.  
 
Even if that's potentially too generous, though, I wouldn't bat an eye at allowing a PC to knock an opponent prone with a Triumph, talent or no.  
 
Talents like these are a slippery slope as far as I'm concerned, as it can potentially result in a problem that plagued 3.x D&D - there were so many super-specific feats, etc. that you couldn't really attempt much of anything because "you don't have that feat."  
 
It's against the spirit of the system to say "you can't knock the target prone because you don't have that talent!" but at the same time why spend XP on a talent for an effect that most reasonable GMs would give you for a Triumph or 3-4 Advantage?