Serenity was a theatrical release. Also while it may have not been your preference, it was IMO amongst the most accessible of entire Firefly series.
First, was the
Serenity that aired on TV the same as what played in theaters? If so, I truly can't imagine sitting through that in a theater. Much the same as for the final episode of
Farscape (which I thought was about as lame as it could possibly have been, in context), I watched whatever it was that aired on TV for finality's sake, but didn't think much of it compared to the (in
Firefly's case, short series of) regular episodes it was nominally based on.
As for my opinion of
Serenity, I wasn't commenting on its "success", but my own opinion of it, which is what this entire thread is about, no? I have no real/firm opinion as to how
popular or financially successful a
Buffy "remake" might or might not be, but given the modern media-watching public's penchant for (what I consider to be largely unimaginative) derivative work, I expect it would be at least moderately successful, if only out of nostalgia among the many of its original viewers who are now entering or rapidly approaching early middle age...
And as for
Serenity, it wouldn't surprise me to find out that it was more "popular" than
Firefly, but as in the case of
Buffy, what I liked so much about the show wasn't the basic storyline, but the characters, their relationships, the acting, and the dialog itself, and what I perceived to be various subtle and not-so subtle subtexts (whether they were all intended or not) as they played out over time. I thought
Buffy-the-movie, for example, was entertaining in a mildly satirical, dumb-but-cute way, but based on it alone, (a) I didn't make a mad dash to see the series when it first aired, and (b) I
certainly wouldn't have guessed I'd end up liking that as much as I ultimately did. (In fact, while I thought the series was "watchable" enough even early on, it wasn't until the latter part of the first season and beyond that I started thinking it had real, long-term potential...)