I'm not sure what tokio (or axum) can do to avoid the trait bounds. Would it makes sense to provide a "share nothing" runtime implementation that can be injected at startup? I wonder how the intermediate layers (e.g. axum) would indicate that futures are usable by a more generic runtime which may or may not need Send + 'static.

Without some way to write generic code for either runtime, the whole tokio ecosystem would end up bifurcated by this choice of runtime.