Another HOT thought: why do we need indcreatexid at all?

AFAICS, the whole indcreatexid and validForTxn business is a waste of
code.  By the time CREATE INDEX CONCURRENTLY is ready to set indisvalid,
surely any transactions that could see the broken HOT chains are gone.
There might have been some reason for this contraption before we had
plan invalidation, but what use is it now?
        regards, tom lane