If you look at the history of infiniband, it was always intended to be something like numalink+xio. Origionally you were supposed to connect peripherals, storage, and processor nodes onto this big network and add and remove them all dynamically. It got scaled down from that, and now is pretty much used as a high-speed network, with the occasional RAID attached directly to it. Numalink can be used in this way too. One does not need to make a single numa domain from an altix.
The numalink chip also has the extended cache directory logic in it, which allows large numa machines. Importantly, that version of large is large on the scale of numa database servers, but rather small on the scale of supercomputers. Even SGI has to fall back to infiniband for the really large machines, such as the two big systems at nasa. It's not as feature-rich as numalink, but it'll scale to tens of thousands of nodes, sorta affordably. I should note that there's no reason that the cache director chips can't talk to one another over an infiniband network. Noone has invented this chip, but the network can be an independant piece.
I agree that SGI has long had great technology, and useful products. (I reserve the term great products, as they have tended to have great strengths coupled with great weaknesses) But I would not say that their products have been successful. If they had, SGI wouldn't have been circling the bowl for the last ten years. SGI learned how to make a lot of money when they were at the top of a growing market. They never learned how to make money in a shrinking market, or how to transition to a profitable spot in a different market.