This isn't your problem, but starting in July, 2019, cars that were produced on July 2015 will have their subscriptions expire. This will continue on as more and more cars hit the 4 year old point. July 2015 is a special date, because i3 production prior to that has 10 years of connected drive.
On an unrelated note, I'm not sure how BMW is going to support 2014 MY i3s until 2024, as they all have 3G modems, and it seems like many wireless service operators are planning on killing their 3G networks. Perhaps they might offer an upgrade to the LTE modem found in 2018+ models, though you would almost certainly have to pay for it. By 2024, many i3s will be on their 3,4, or even 5th owners, and said subsequent owners may not care too much about connected drive. I think part of it also depends on how much more i3s depreciate. It seems to have hit a floor of around $15,000.