With a solid LTE network, Sprint should have no problems maintaining truly unlimited smartphone data for a long time...even with partners, MVNO's and prepaid divisions. Especially considering the deep spectrum resources that Sprint can exploit in Clearwire.
However, unauthorized tethering will likely start being cracked down on. AT&T now sends messages to users devices when they are doing unauthorized tethering. I expect Sprint to first crack down on heavy unauthorized tetherers, and will likely then move on to crack down on all unauthorized tetherers if they have to. Using LTE as an unlimited home ISP is not going to be allowed by any network. We don't have enough spectrum to do that except in probably rural and remote tertiary markets.
The people who have enjoyed that in the past just need to be happy for the years they were able to do that. It's just a luxury that will not be sustainable in the future.
Robert