`background:true` is more sensible default for `createIndex()`

Per https://docs.mongodb.com/manual/reference/method/db.collection.createIndex/#options-for-all-index-types, the `background` option defaults to `false`. This has caused operational problems on our team using MongoDB. We've forgotten `background:true` and it's blocked all database calls, causing downtime (until we could run `db.killOp()`).

Wouldn't `background: true` be a more sensible default? I can't think of a reason we'd ever use the default that blocks all database operations. Is there a historical reason this isn't the default?