I think I would be fine with a compendium for different groups of armies, even if they can work together as well. (As long as it's group with some sense the timeline/story). I think they need to do something to increase the rate at which books are updated/faq'd so that they don't lose players that love their armies due to the armies suddenly becoming bad. (Terrible or unplayable depending on who you ask) I've played the game since 5th ed and I've see all kind of good changes, but some armies always suffer the last army syndrome. By the time they get a book, they are good for about a month then the news of the newest edition comes out and they change the rules slightly (or a lot) and that army that so many people have been waiting on, falls on it's face after only a month.
Having been a long time WE player, as well as Beast of Chaos/Daemons of Chaos (Yes I listed them that way because I was rather irked when all of my daemons models were unplayable because I didn't have enough for a stand alone army), O&G, Vampire Counts, and recently dabbling in Empire. I've seen many of these armies get tons and tons of updates, new models, love in general from GW. The the former two armies, get trounced in the mud. I hope a change to a compendium system where they could reasonably update some 4 book to cover all the armies would fix their issues I've seen develope of the years.
p.s. Have you seen how many models in the WE line are still metal? Core Metal spearmen...........sigh.