Well, there's a simple way of seeing why they'd change them.
Eldar never had them and didn't need them. No optional rules.
Chaos never had them and didn't need them. No optional rules, but an assload of codex printings.
Tau never had them and didn't need them. No optional rules.
Specific space marine armies used to need them, but with their own codex they don't. No optional rules.
Dark Eldar and Orks haven't seen the light of day. DE won't need them. Orks might be improved by them, but without seeing the rules nobody knows.
Space Marines really could use them, to provide some difference between the players of this most popular army. However, now that there are a variety of noncodex armies for the really big divergences, the current doctrines could feasibly be contracted until they were functionally contained in the basic codex.
Imperial guard, though, really do need them. They're the only effective means thus far of giving the players the full scale of options. It is possible that they could be folded into the main rules, but that would take a great deal of effort.
This is the problem most of you seem to be having in your thinking. They will not take the old codex, cut out the doctrines section, and call it fourth edition. IF the doctrines go, they will be replaced by something similar, or their rules effect will be folded into the main army.
Personally, though doctrines are a pleasing way of making armies diffferent, they're easy to exploit and are almost always used in ways counter to their original intent. Originally, you were supposed to pick some while making your army to make it more appealing to you. However, most people have just seen them as an extra layer of strategic advantage, and pick and mix per game for whatever help they can get. As such, either doctrines will stay like this, or be changed to fix it.
If they stay the same, they are effectively more codex options. In line with GW's policy of streamlining and many options, then they'd be slung into the main codex. The difference would be minute, though nobody would see it that way.
If they change, they'd have to be in some way permanent for the army, or make sense to be changeable. High strategy and military organisation doesn't change from fight to fight, so that doesn't work. It'd either see srict and seemingly arbitrary rules to lock down the doctrines, or the general effect of doctrines weakened and most of them folded into the codex.