Newer cellular technologies are released by cellular carriers to support customers with their needs to stay connected. Older cellular technologies are being replaced by newer technologies, and everyone who owns a smartphone is aware that the development of smartphones is in balance with the introduction of new cellular technologies
Car manufactures have proudly introduced connected cars last decade. However car manufactures like BMW don't seem to understand that introducing cellular technogies into cars does make a car manufacturer responsible to keep the cellular technology of the car up to date to make sure the car will stay connected during the whole life cycle of the car. Qualcomm chips  used in BMW telematics units do have compatible LTE (4G) versions since 2010. So today there shouldn't be any problem regarding the 2G/3G sunsets at all.
With the upcoming 3G sunset BMW is taking a different path. The same path which BMW took with the 2G sunset. BMW seems to feel limited responsibility and the cellular sunsets are, according to BMW, in the first place a problem of the person who owns the BMW.
The decision to phase out 3G network technology was made at the discretion of the respective cellular carriers and lies beyond the control of BMW. 
However, some BMW owners are lucky in case BMW does offer a free technology upgrade. Others are abandoned by BMW. BMW isn't clear why two cars which share the same technology and Integration Level (iLevel) can't be upgraded both with new technology. The manufacturer (Valeo / Peiker) of the telematic units has done an excellent job, because the telematic units are either exchangeable or upgradeable  by exchanging the riser board of the telematics unit as a low cost solution to support newer cellular technologies like LTE.
Based upon pure facts it's odd why BMW AG isn't supporting BMW owners with upcoming 3G sunset. BMW is the manufacturer of the whole vehicle and has a responsibility too. Cellular sunsets are planned into the future by the cellular carriers, so it's weird that BMW AG is clearly hiding behind the decision of cellular carriers. It's very weird that BMW has declared that the situation is beyond the control of BMW AG, because from November 2016 (16-11) 4G is supported with the iLevels and required technology upgrades do have a part number at BMW and can be ordered and activated with ISTA (module replacement) by any BMW service organisation (SIB 84 02 21). The old telematics unit (TCB/ATM/Combox) has to be exchanged with ISTA module replacement procedure to make sure the VIN of the new telematics unit does match with the VIN of the BMW and the correct IMEI is registered at the BMW back end to enable ConnectedDrive services for the new 4G telematics unit.
A 4G TCB/ATM might need a software update  in case it's affected by the 3G cellular technology discontinuation. Certain ConnectedDrive services for this vehicle, including BMW Assist eCall, will be affected and no longer be available at that time. This vehicle is eligible for a one-time software update to keep these services active. (source: SIB 84 02 21)
*Do share the same cellular technology but are missing from the overview (SIB 84 02 21)
|84 10 6 836 777||TCB 4G Trunk Mount*|
|84 10 5 A0F C23||TCB 4G Trunk Mount|
|84 10 5 A0F C29||TCB 4G Trunk Mount|
|84 10 9 843 931||ATM 4G Roof Mount|
|84 10 2 447 925||Combox Next 4G Trunk Mount|
Conclusion: It's clear that BMW actively avoids responsibility with Cellular sunsets and most likely the only reason is that BMW wants to sell new BMW's only.