At 1Gbps (125MB/s), a gigabyte takes 8 seconds to download, less than most AAA games take today just to load from disk. There are videos of Google Fiber users getting ~40MB/s downloads from Steam, so I wouldn't be surprised if at least some users reach that in the near future. And if you rely on a fast internet connection, you only have to send the assets necessary for one level at a time.