It's a shame because it seems to me like RxJava is eclipsing what was previously a pretty fertile ecosystem of Actor based approaches, but RxJava itself is not very nice to use compared to those which are much more natural.
Loom will present an ergonomic alternative for concurrency that is strictly superior to futures, but it will not replace or significantly simplify RxJava[1].
Project Loom is also orthogonal to actors - it really doesn't dictate any specific concurrency model, it just provides fiber support in the JVM. We will definitely see actor libraries once project loom is released, but these are already available now, and they are not widely used. The only thing Loom can help them with (I hope) is better performance, but I don't think that's the reason the actor model lost ground in the Java world.
[1] Project Loom can be the basis for some minor simplifications, as JetBrains did in kotlinx.coroutines.flow.