Too many WiFi networks in the same range can indeed cause interference issues. See how well the Wifi works on your smart phone the next time you're at something like a State Fair or large festival
That's not an issue with wifi interference, that's simply a bandwidth issue. Lot of people trying to use a system designed for less demand will bring a system down. Imagine a NASCAR event. The town usually have 50,000 residents and then 75,000 NASCAR fans show up. That tower is simply not going to be able to handle the additional volume. Since Sprint sponsors NASCAR, they also bring into additional mobile units to help handle the volume. Not good if you are a Verizon customer but it sure makes Sprint look good!