Yeah, the addition of new recordings - both new to the world, and old recordings whose entry is new to musicbrainz - would be among the cases with respect to which I'd expect to see the most activity.
I think I get your point, though some of the numbers @andreivolgin shared later in the thread, and some of my own direct experiences with collection subscriptions seem somewhat inconsistent with your premise. Regardless, I'd suggest that frequency of change shouldn't necessarily be directly correlated with the importance of timely notification of change.
If you'll allow me to add to your inventory of inexact metaphors: Imagine a University creates an emergency alert system to warn folks on campus of dangerous situations. Perhaps it's an exceedingly safe campus, and occasions for usage occur only once every few months. Surely you'd not argue that the infrequency of emergencies implies that notifications of the few that occur can lag by several months, weeks, days, or even hours.
Though we're obviously not dealing with issues of public safety, users do typically have fairly high expectations for product quality, data integrity and freshness, etc. Though we don't all have the resources of Google, Facebook, etc., the experiences users have with products developed by those big companies set the bar pretty high for the rest of us.
If I'm developing a product that relies on users trusting the quality of data, then of course I'd want to offer them the best data available, as close to as soon as it becomes available as is reasonably achievable. And if I'm using musicbrainz, and I tell my users I'm using musicbrainz, and they go add new data directly to musicbrainz, and it doesn't show up in my system for several days, that reflects poorly on my product.
In any case, I feel like most of this discussion is somewhat moot. If it were exceedingly complex to support my use case, or my intended usage was going to produce excessive strain on musicbrainz resources, I'd understand the hesitance, but unless I'm really off base in my thinking, I believe the options I have in mind would reduce the strain, and at least some would be relatively simple to implement (and as alluded to elsewhere in this thread, possibly contributed by me).
If you still need further elaboration, after reading the above, ping me back and I'll try to outline some sort of example data flow ... or something. I'm fully invested in making sure I'm making myself clear!