I would say it is not a replacement but an addition.

IP isn't going anywhere any time soon, but we add two capabilities on top. The ability to dial an endpoint by key, and the ability to get direct connections whenever possible.

That being said, if some other technology becomes popular that actually replaces the IP address paradigm, iroh is well positioned to make use of it. From the point of view of an iroh application developer nothing would change. You still dial by key, and iroh will just make sure under the hood to get you the best possible connection, IP or otherwise.

A little bit of both. Natively it relies on QUIC and leverages existing IP infrastructure, however it also works with custom transports just as fine so you can interact via bluetooth for example.