In a recent conversation I had, there was a disagreement regarding the definition of Functional Harmony. As I've understood it, Functional Harmony would be defined by chords having a function and is generally distinct from Modal Harmony, Atonal Harmony and Non-Functional Harmony (obviously). We were debating whether the classic I-V-vi-IV pop progression would be considered functional or not. In my thought process, this wouldn't be functional, as the actual functions of the chords are never fulfilled: the dominant chord never resolves to the tonic; the predominant chord never precedes the dominant chord. They argued that the dominant resolves to vi, which is a tonic functioning chord, giving a deceptive cadence, and the IV resolving to I gives a plagal cadence. In my estimation, the V-vi relationship doesn't really feel like a cadence, as it's in the middle of the phrase, but even if it were thought of this way, the progression would still never be giving the true V-I cadence; resolving to a tonic functioning chord alone does not satisfy the dominant to tonic relationship. So I would think of this progression as being more so a Modal progression, most commonly Ionian mode.
In short, does Functional Harmony require that the Dominant to Tonic relationship be fulfilled and, if not, what are the boundaries of Functional Harmony?