For the vast majority of code that runs on either desktop or mobile, the latest and greatest hardware features don't matter, and neither do arch-specific optimizations. Startup costs are trivially solved by an OS-level precompilation and caching mechanism. And we already have sandboxes for apps, even on the desktops (both macOS and Windows store apps). Besides, why can't WebAssembly have a different, less limiting sandbox outside of the web?
The real benefit is having a portable bytecode that 1) can accommodate any language out there without constraining their object and memory model, and 2) is guaranteed to be compiled to native code by an optimizing compiler when deployed. Add some basic APIs, and we could have a situation that has heretofore has been enjoyed pretty much only by x86 - the ability to compile code, and run it 30 years later as is.
The real benefit is having a portable bytecode that 1) can accommodate any language out there without constraining their object and memory model, and 2) is guaranteed to be compiled to native code by an optimizing compiler when deployed. Add some basic APIs, and we could have a situation that has heretofore has been enjoyed pretty much only by x86 - the ability to compile code, and run it 30 years later as is.