It seems like Slingbox doesn't have a NAT-proof solution, and has been using their servers as reflectors, which suggests there's an ongoing opex cost for the bandwidth and maintenance that isn't sustainable.
If they'd charged a subscription fee they might have more incentive to run the servers. Too bad their focus was on "no monthly fees"!
Dish also might have an upcoming/future motive to get Slingbox users to replace it: