That might be true. C++ is used because there is existing code for the planes we will see in the near future and the developers already know it.
But i guess also because the code is not compiled and would be easy to just copy the plain JS code.
We will see in the future and im looking forward what you can do with it in terms of optimisation, i hope a lot.
But right now, its not fun to use all this advanced G3000 and the FBWA320 for me personally, because the performance is subpar. Even with a decent system and very optimised settings.
Also, i GUESS (not sure on this one) that WASM is not accesible plain code so its not so easy to just copy the custom code. That might be another reason that payware-developers will not use JS/HTML. But if im wrong on that, feel free to correct me.