I'm content to take your word on 720. To be honest I can't claim I would have come to that conclusion by myself. Nor did the ~10 engineers I asked to look at the documentation. Perhaps it's obvious to ES team member but harder for someone outside ES to understand. An enhancement to the documents might be worthwhile.
I think a custom routing rule based on this knowledge would end up being dangerously brittle
I'd be interested to hear more on this. The brittleness I'm considering is if a shard (and it's two replicas) goes offline. Then anything being read or written to it, by the routing rule, is in trouble. Are there any other concerns / dangers?
The temptation is that, for 50% of our traffic, we can drop the search request cost from 90 shards to a single shard. Which would help with our CPU usage.