3G is not obsolete

The US is a special case, for three reasons:

1. It rolled out GSM as a replacement for D-AMPS, rather than as a network in its own right.
2. When it was rolling out GSM, it did so in the full knowledge that it was going to want to upgrade GSM cells to UMTS in the foreseeable future, and put in suitable backhaul links.
3. US consumers have generally been able to afford to upgrade from GSM-only devices to GSM + UMTS dual mode or GSM + UMTS + LTE triple mode devices.

This isn't true in most African countries, or in India - GSM was rolled out as "the" local network, and no forethought was put into building an ATM network suitable for UMTS speeds (as UMTS often didn't exist when these networks were planned). In contrast, LTE just needs an IP network (no circuit switching in LTE), and is thus easier to deploy if you've not built a fast circuit switched network based around ATM technology - Ethernet is cheap to add as an overlay now.

On top of that, Jio's deployment of an LTE-only network in India is putting market pressure on cellphone companies to sell cheap LTE phones - there's no equivalent pressure for UMTS devices, as everywhere with UMTS either has GSM or LTE, so if you're going cheap, you simply remove UMTS capabilities. You can now buy (wholesale, for rebranding) LTE-only devices for comparable prices to GSM-only devices.

Finally, LTE is designed to be much easier for the operator to scale quickly - unlike UMTS and GSM. Thus, if you're deploying today, you can deploy an LTE network that's not that great, knowing that you can quickly scale the bottlenecks up as and when you have customers to pay for it, instead of having to correctly design the network for future demand.