In Switzerland, we already switched to MediumFast due to congestion in cities. It was a coordinated change and real world tests showed no significant worse mesh performance for most nodes. Meshtastic is evolving fast and I think it does become more and more viable as an off-grid or doomsday communication. It also is hard to censor which could be useful for journalism and free speech. Hence, not a substitute for the internet, but a more and more viable solution for many.

Can't speak about any mass gatherings or protests, but haven't had any issues so far with mine. Even in big cities, air util and ch util is below 35% so there is a lot more space available.

By default it implements rough limits that you cannot exceed to make sure that you do not not transmit too much noise. Additionally, you can always establish private channels for your nodes and / or not retransmit at all.

Meshtastic isn't intended for mission-critical uses or as an internet substitute. It is intended for very basic text based communication (e.g. between your friends) or remote IoT devices.

The congestion argument also applies to all radio based communication, there are always people transmitting with high gain, noisy outputs or spam.