I remember back in the 90's, when Intel was going to introduce a unique identifier in their processors that could be used for authentication purposes, it caused a huge uproar and Intel backed down. [0] Having a hardware embedded unique ID that can be (is) used for tracking a known persons location at all times is probably the most intrusive privacy violation we've ever had. That we willingly carry these things around…
The IMSI identifies the subscriber (and/or the SIM card) even if you change phone.
As long as people are able to call and be called and as long as people need to be billed there has to be a reliable form of identification.
This is not an "intrusive privacy violation", it is, as you point out, a technical requirement of our willingness to be reachable and most people think that the benefits far outweigh the very limited drawbacks.