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So currently we have the indexes for rules and we know that codexes are coming. But do you think that every current army will get a codex or will some be folded into other codexes? And if so, who gets the axe?

it can go a few ways:

1) If it's like AoS, new factions will get proper codecies as they bring them out, while old factions (current factions) are left behind gradually.

2) They update each of the factions with a new figure range and codex to further their rules, and bringing out a couple of new ones along the way (like the Primaris and Death Guard). Possibly some of the less popular ones get the axe, like Tomb Kings and Bretonnians in AoS did.

Those are the two options I see, and it's hard to tell which could go down. In AoS, we mostly see new factions being announced and released, and they're the ones who get the new specific army books, while the older factions aren't updated, as the new ones essentially replace them in a way, while leaving the older ones in there if people have a big collection. Similar to Primaris Marines and vanilla Marines: it's a new faction based off an old one, but with updated figures and an army book.
Who knows if that'll be the case in 40k, just my predictions.

They have said in Warhammer Fest that there are 15 books in production for 8th edition. They didn't said if those were all codex, but one can assume...

To answer the question: 'Everyone' includes SoB so...no.

I think that factions will remain pretty much the same, but for the comming year, we will have an almost exclusively Primaris Marines release. They will get their codex and their new models released first. The Death Guard will probably have its codex and a few model released relatively quickly. The new Eldar subfaction might also have its first exclusive units and codex if we get some time. Expect the Skitarii and Cult Mechanicus to be roled in a single Codex in the future. I don't think we will see any army being discontinued. I won't hold my breath for a release of new Imperial Guard, Sister of Battle or Dark Eldar models and rules. Regular Space Marines will never see another release as they will slowly be phased out by Primaris Marines of their particular flavor.