Its on it's way there. Right now, renters that reach 5 Terabytes of uploaded data might see their clients slowing-down or even freezing (lower amounts are ok). For the next update (end of September), the optimizations should be able to allow 40Tb or even more, as the whole database system of renters is being redesigned. Next update is totally focused on the renter side of Sia. There are plans to keep increasing this number in future updates.

Actually this is not a problem of the protocol or the nature of the network, it is just an optimization problem of the reference Client. Either the client of the Core Team or clients of other developers should be able to keep increasing the performance.