You don't, just, easily rewrite everything.
Being able to just re-use is the trick!
Why would you rewrite (parts of) Cargo from Rust to something that runs on the JVM, when you can use Wasm as basically an intermediate target to compile the Rust down to JVM bytecode?
Or how about running something like Shellcheck (written in Haskell) on the JVM as part of a build process?
You can see the same idea for the Go ecosystem (taking advantage of the Go build system) on the many repos of this org: https://github.com/wasilibs
Yes they each have their own linear memory, that’s one of the advantages of component model. It provides isolation at the library level and you don’t have to implicitly agree that each library gets the level of access your application does. It provides security against supply chain side attacks.
Having said that, component model isn’t supported by all runtimes and since its binding and code gen are static at compile time, it’s not useful for every situation. Think of it like a C FFI more than a web API receiving JSON, for example. Upgrading the library version would mean upgrading your bindings and rebuilding your app binary too, the two must move in lock-step.
From the perspective of a Bazel ruleset maintainer, precompiled helper tools are much easier to provide if your language has easy cross-compilation. So maybe one day Zig will start to make an appearance too.
And in terms of design, they're closer to COM or OLE. The modern replacement for CORBA/DCOM/etc is HTTP+JSON (or gRPC), which doesn't try to abstract away the network.
The best way I think to describe WIT is that it seems to be an attempt to design a new ABI, similar to the System V ABI but capable of representing the full set of typesystems found in every modern language. Then they want to use that ABI to define a bunch of POSIX-ish syscalls, and then have WebAssembly as the instruction set for their architecture-independent executable format.
The good news is that WIT/WASI/etc is an independent project from WebAssembly, so whether it succeeds or fails doesn't have much impact on the use of WebAssembly as a plugin mechanism.