It is the most consistent with the general design of the system, for reasons that are pretty deeply baked into the system. I'm reminded of the fact that REINDEX CONCURRENTLY's completion became blocked due to similar trepidations. Understandably so.
I may mistake, but I recall the fact that all indexes builds started during some other (long) index build do not finish with indexes usable for selects until that long index is built. This may and may not be a source of amcheck misbehavior. Just a note what could be possibly considered.
