RdC wrote on 01/07/20 at 22:40:46:
To what extent would there be general agreement that the only viable lines of the Modern are the Tiger treatment with .. a6 and the Hippo treatment with the double fianchetto and playing Nd7 and Ne7?
Ugh. Assuming white goes for either the Averbakh or Austrian setups, I have doubts that Tiger's ...a6
is viable. At the least it is risky. Certainly Tiger agrees that against the Averbakh black must transpose to the King's Indian. I haven't studied Tiger's lines against the Austrian (I don't play that way as white), but I did look superficially at his analysis, and the whole thing looks dodgy. Maybe with perfect play black can equalize, but I wouldn't bet on it. Transposing to the Pirc and playing 5...c5 looks solid enough, maybe just not enough winning chances for Tiger?
And against those same two setups, without doubt the double fianchetto (so-called "Hippo") is
not viable. Experimental at best. Black is not equalizing, and might get blown away by resolute play.
Against simple classical play -- white holds back both the f-pawn and the c-pawn -- it's a different story. But in that case black has a wide choice of viable defenses.
Not directly related to your question, but black doesn't always have to play ...Nd7 and ...Ne7 in the double fianchetto.