There is a certain mystique surrounding the undefeated top-class horse, especially when this horse has won all its races impressively. And when this horse posts an equally impressive BC Classic win as its swan song...then the mystique can't help but grow, IMO. I don't know if Flightline belongs among racing's "immortals"...I leave such determinations to the "historians" of the sport. As a horseplayer...I am content with my opinion that this horse was the most talented of all the horses that I have ever seen in my 42 years of participation in the game.
When judging the quality of a racehorse...we use "brilliance" and 'consistency' as measuring tools. And Flightline had tons of the former, even if the latter quality was left in some debate due to the economics that govern today's game. In an imperfect world...that's good enough for me.