>> 4.) It only works on GSM, i.e 2G. Hardly anyone uses that these days. (I don't know if a similar passive approach would work on 3G and 4G, it may be that only software is required in order to support more than 2G).

Do you have some data that supports this claim? I haven't gone sniffing recently- but I believe T-mobile is still wed to 2G for the foreseeable future, and internationally, 2G is still in a lot of countries. I believe one northern european telco is going to decommission their UMTS network and support both LTE & GPRS.