At any rate, this is the world that we live in. Advise your non-tech friends to run updates to get the latest microcode and software mitigations. Install uBlock for them and block possible attack vectors aggressively (ads, trackers, etc). As a technical user, it's best to disable JavaScript completely by default and enable trusted third party JavaScript using e.g. uMatrix. Of course, this has other benefits too: creepy companies don't get to follow you around.
All depends on your threat model; to take one extreme, if you're crunching numbers with your own optimized numeric code, the issue of untrusted code of "random JavaScript off the net" is not an issue. And I doubt many mainframes are used to casually browse the net ... I certainly hope not at the same time they run the night's financial transactions!
In a dream world. It's become obvious that our general-purpose systems are far too complex to be proven secure, and trying to run untrusted code on such a foundation is going to turn up vulnerability after vulnerability after vulnerability. It is madness.
The "elephant in the room" with all these attacks starting from Spectre/Meltdown is that an attacker has to run code on your machine to be able to exploit them at all.
To the average user, the biggest risk of all these side-channels is JS running in the browser, and that is quite effectively prevented by careful whitelisting.
As you can probably tell, I'm really not all that concerned about these sidechannels on my own machines, because I already don't download and run random executables (the consequences of doing that are already worse than this sidechannel would allow...), nor let every site I visit run JS (not even HN, in case you're wondering --- it doesn't need JS to be usable.)