Maybe we should just give up wanting to run untrusted code safely as a failed experiment.
For example 90% of the JavaScript I run is for advertizing purposes. The pragmatic solution would be that either everybody moves to self-hosting ads, or that the neccessary telemetry for third parties is moved into a browser function. Web pages (that are not applications) should not need a "turing complete" language (in the vulgar sense of the term).
If you have real applications - like GMail or Facebook - you should run them in a sandbox, but they should not be treated differently from native apps. Yes, a native app could use Meltdown etc. to attack other apps, but usually it can compromize you even simpler since most desktop apps are not sandboxed. It can just open the memory of other apps or abuse accessibility functions. My point is, I don't expect MS Word or Photoshop to be perfectly sandboxed. I explicity trust the vendor when I install their app. If we would really trust what we trust, and really distrust what whe don't, a lot would be won.
Of course, I know that the world is not set up rationally and we'll continue to be compelled to trust untrustworthy code on our PCs, so for the time being it looks like we're stuck with the mitigations.