Mobile code was dying before, but the final nail in the coffin is the Spectre/Meltdown class of vulnerabilities (with new variants coming out monthly).
Is it though? I mean, has anybody stopped using Javascript as a result of Meltdown/Spectre?
True, Java applets largely died off, but that was as much because it took Sun WAY too long to ship the "Consumer JRE", coupled with a rash of Java security issues over a certain window of time, that made people scared to enable Java in their browsers. I doubt many more people disabled Java after Spectre/Meltdown, than before.
Couldn’t one run user scripts on separate servers with non-OoO CPUs: low-end ARM or MIPS or RISCV or whatever?
It seems modern microprocessors simply aren't designed to run untrusted code.