Adding those features doesn't mean you have to start using them for everything. This is different from how they work in the JVM or CLR- their role in WebAssembly is to enable interop with the host (which already has them regardless), and to round out some of the capabilities that native code has but which can't be implemented efficiently on the current WebAssembly sandbox.
I don't see how that harms WebAssembly as a lightweight cross-platform target. It may make it more difficult to implement, but AIUI there will always be room for simpler hosts that leave these features out or keep them simplistic. There is no equivalent ecosystem problem to adding these kinds of features to a language, where all your dependencies will start using them, any more than inherently exists for native code that can already just implement those things itself.
I don't see how that harms WebAssembly as a lightweight cross-platform target. It may make it more difficult to implement, but AIUI there will always be room for simpler hosts that leave these features out or keep them simplistic. There is no equivalent ecosystem problem to adding these kinds of features to a language, where all your dependencies will start using them, any more than inherently exists for native code that can already just implement those things itself.