So recently Eldar, Imperial Knights, and now Space Marines (even new rumors of Dark Angels too) are getting new codex's despite their previous codex not being that old. I've seen a number of posts where people are concerned about the product life. I'm not sure if this cycle of a new codex every year or so will be an issue. I think it's more of a 'GW found a style they like' thing. The recent configuration of the new codex's is far different then the ones released just last year. I think the people at GW found the style they like and are working to bring all codex's into line.
I'm not too concerned about having to replace my Eldar Codex in 2017, I'm looking forward to finally seeing Codex: Eldar Exodites (afterall why use the word 'Craftworld' in the title). I think once GW redoes all the Codex's in this new style, they will focus even more on the other sub armies like they have been doing recently.