That's also a problem that it'd be fairly straightforward for Google to solve by automatically spinning up smaller, entirely separate serving clusters for customers who are worried about such a blowout (for a fee, obvs). It's just the serving tree (+ whatever in-memory storage service they use to do distributed joins nowadays), no need to duplicate the rest of the service. The caveat is, a smaller cluster will favor query optimizations specific to that smaller cluster. Some of those "small cluster" optimizations could hurt query performance when deployed against BQ proper with its tens of thousands of workers.

Also, BQ does explain the query plan to some extent: https://cloud.google.com/bigquery/query-plan-explanation. Not quite at the level of a "regular" SQL DB, but it does give you some info to work with when optimizing queries. If you haven't used it in a while I'd give it another try.