It may seem to you like the market for expository papers has narrowed, but I think it has increased tenfold. Personal blogs appear to be the medium of choice for exposition these days. I am sure you are aware of this (in part because many high-profile Mathoverflow users have popular blogs), but I just wanted to remind you that blogs can reach a wide and interested audience. Perhaps not everything belongs in a journal.
EDIT (Douglas S. Stones): Adding to the "alternative to journals" answer -- authors could consider writing a book instead.