Birds of Paradise are ok as a budget replacement for Hierarch. Loosing the exalted trigger will hurt you from time to time but it should be manageable nonetheless
Definitely, I've been toying around with bant spirits since spring or so (before the new lord pushed it to t1) and I had 1 hierarch and 3 birds. While hierarch is better birds are a very solid replacement. Same thing for the manabase cavern is good (vs control of course) and canopies help you a bit if you flood but is still playable without.
The "problem" with KCI is that plays a different game form almost any other deck in the format. The usual maindeck choices of midrange and control decks don't line up well against it and trying to fight with 2/3/4 of sideboard cards is not enough to disrupt it. The high learning curve keeps it in check for now but I am afraid of what the meta might look if it becomes more popular...
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Definitely, I've been toying around with bant spirits since spring or so (before the new lord pushed it to t1) and I had 1 hierarch and 3 birds. While hierarch is better birds are a very solid replacement. Same thing for the manabase cavern is good (vs control of course) and canopies help you a bit if you flood but is still playable without.
1