Atheros thinks single-stream 802.11n has potential to replace 802.11g: Atheros has introduced the Align, a family of chips that use a single antenna to bring some 802.11n advantages without the spatial multiplexing, improved receive sensitivity, further transmit range, or antenna diversity, among other characteristics. The company told EE Times that they wanted to get beyond 802.11g for future devices to bring the advantages of newer designs. This should allow G prices with some improved N features.
This won't break 802.11n compatibility, as 802.11n can hear a single spatial stream just as well as it can multiple ones. In fact, 802.11n provides the flexibility to have multiple streams sending the same data redundantly, which is what Quantenna has opted to do with its consumer gear--sacrificing raw speed for resilient performance.
Atheros is claiming 50 Mbps in TCP throughput with 20 MHz channels and 107 Mbps with 40 MHz. This isn't out of line with the base raw symbol rates in 802.11n (65 Mbps instead of 54 Mbps). TCP throughput still has overhead, of course, so it's likely that single-channel N will be about twice as fast as the 20 Mbps or so 802.11g could achieve.