I don't see now in this day & age why when you pay $60 a month for 200gb of data on a 4G wireless modem that you don't offer customers the option of rolling over the data they don't use? Most companies do.
Perhaps I'm confusing the service you are talking about. Mobile Broadband generally doesn't have a rollover option from anyone?
Optus - No
Telstra - No
Vodaphone - No
TPG - No
Amayasim - No
Exetel - No
PrePaid Mobile generally has Roll over but costs more and has lower data limits eg. Boost - Yes (but $60 buys ~70Gb)
Do you have an example of a Telco that does provide what you are asking for?
FWIW the 4G networks have been oversold already on mobile broadband capability. 100Gb+ Plans put a lot of strain on the capacity. Allowing people to store up even more data would probably not be able to be supported by the infrastructure.