A valid response is that as soon as a data service uplink is provided, then common IP routing via the GGSN could take over, and everyone could be using messenger apps. D-GSM (btw not limited to 2G) is indeed most interesting for routing "old style" voice calls and SMS. These are still in wide use, and arguably more stable than pure data links, which have to compete for bandwidth with other data (base stations can actively prioritise voice).
If you think LTE (4G), then everything is already a data link at first sight; but most carriers still use CSFB, where the voice calls fall back to a 2G RAN.
The lack of open public spectrum is a clear mistake of the governing authorities. To run a GSM+3G+LTE network for the annual CCC congress, we have to ask commercial operators for ARFCNs to use, every year all over again, and every year it is unclear what the response will be, often up to very near the build up of the event. If we had a publicly usable spectrum for 2G,3G,4G (and 5G?) here in Germany, so much more innovation and free/open infrastructure projects would happen in mobile communication. Network coverage holes are high up on the news these days, just imagine if locals were allowed to help themselves and not rely on profit-only oriented commercial operators...
re "strange to choose 3g": as some operators even phase out 3G completely, we can expect 3G base stations to be available cheaper than ever. That's only one of the reasons: Osmocom started out from the security analysis perspective; having 3G openly available is a benefit to the general public as well as small business in various ways.
(The opening lines of https://osmocom.org/news/30: "3G is dead, you may think. From the perspective of large scale operators, that may well be the case, but this is precisely the reason why Open Source support for 3G is becoming increasingly interesting: when the focus for earning money shifts towards LTE infrastructure, the threshold for setting up 3G networks is becoming easier to surpass for everyone else.")
If you think LTE (4G), then everything is already a data link at first sight; but most carriers still use CSFB, where the voice calls fall back to a 2G RAN.