I believe that the most important property that a virtual ISA such as WebAssembly (or JVM, CLR, …) would have for cloud application developers is not to remove the need to recompile but to remove the need to test the code on each type of hardware that it is supposed to be deployed on. This determinism is something that has hitherto not been available for C/C++ developers.
If WebAssembly/WASI would develop in the right direction, I think that it would not pose a threat but rather help to reduce the cost of adopting new types of hardware, which would allow them to better compete on just such aspects such as performance/Watt where The Mill would excel.
Even though WebAssembly has limited performance (as it has been designed for fast code-generation rather than fast code-execution), I think that could be a lesser concern as a lot of an application’s run-time is often spent in imported modules anyway — which could still be in native code. But WebAssembly is also evolving.
I don’t see advantages of using WASM on the server/desktop for anything but C/C++ code though, but the Rust community seems to also have adopted it for some reason… Maybe just hype?