Well, Bryn, I'm only pointing out that the fact that a piece of music develops almost imperceptibly for an hour - and requires careful listening to detect that - would not
in itself (or, as I said,
automatically) make it engaging
as a piece of music ...

notwithstanding your reasonable point that the almost imperceptible development does free the piece in question from the charge of being endlessly repetitive.
I intended my comment as a leading on from, rather than an objection to, your comment; an almost imperceptible development of the argument, perhaps.