Monarch can support both ad-hoc and periodic, standing queries of arbitrarily large size, and has the means to spread the computation out over many intermediate mixer and leaf nodes. It does query push-down so that the "expensive" parts of aggregations, joins, etc., can be done in massively parallel fashion at the leaf level.

It scales so well that many aggregations are set up and computed for every service across the whole company (CPU, memory usage, error rates, etc.). For basic monitoring you can run a new service in production and go and look at a basic dashboard for it without doing anything else to set up monitoring.

That's a choice Prometheus has made, not an invariant. Many scalable systems support arbitrarily large queries as they themselves scale. Prometheus pretty much prevents creating arbitrarily large datasets to begin with, so the point is kind of moot.

Requiring query authors to understand the arrangement of their aggregation layer seems like a reasonable idea but is in fact quite ridiculous.

Also, very large ad-hoc queries are supported, with really good user isolation, so that (in general) users don't harm each other.