Ad blockers primarily look at domains, so blocking will continue to be possible at the request level. They aren't interpreting or parsing JS to begin with.
> - stronger DRM
If sites were going to ship Web Assembly-based DRM, they would already be shipping Web Assembly along with the Emterpreter. Remember that wasm has a polyfill already. I haven't seen that happening, so I see no reason to believe it'll happen in the future.
> - Bitcoin mining that regular user can't detect
A regular user certainly would notice the 100% CPU consumption. And anyway, bitcoin mining in a WebGL shader would be more profitable than anything wasm-based.
Moreover, though, surreptitious bitcoin mining on consumer PCs would be ludicrously unprofitable no matter what. Here a Stack Overflow answer from last year that calculates how much a site with 2M daily visitors would make if they could all somehow run the fastest C implementation [1]. It was less than 50 cents a day back then, and in the meantime the hash rate has grown by nearly an order of magnitude [2]. Good luck.
[1]: https://bitcoin.stackexchange.com/a/42413
[2]: https://blockchain.info/charts/hash-rate?timespan=2years