There are oodles of games that started as free games and then made a commercial release later on. It's not really unique to web or "instant". Further, there's

There are oodles of games that started as free games and then made a commercial release later on. It's not really unique to web or "instant". Further, there's a huuuge dropoff in interest once you do go to a paid release. Often these games are accessed from locked-down school or office environments as part of a "game jukebox" in the same way that YouTube often doubles as a "music jukebox" - there's minimal investment in what you're playing, so, like fast food, it's more important that it leaves a good impression in five minutes than to actually have substance. FNF's basic appeal is akin to a fashion brand with a cool logo - these characters could have used any number of delivery vehicles but the game, and the game when put on Newgrounds specifically, happened to be the right one with the right audience. And because it's positioned for ease of access you get the high virality. When you flip things so that the audience pays upfront, they have to have some conviction in this one, out of a countless number of games, being the one they should champion. It's just a radically different proposition and only some games can cross over between the two markets. FNF does benefit from having a lot more hardware and bandwidth available these days. Music is classically the Achilles' Heel of web gaming because it's either space-inefficient or you need to invest in a sequencer format of some sort(which was only feasible in Flash by fighting the available technology every step of the way). But FNF delivers full-length audio streams without too much difficulty, so we've clearly made some giant strides there.