No constant addition and removal of new APIs. Lockdown of special purpose APIs with explicit user request to unlock: 99.9999% of sites will never need WebGL, 99.9999% of sites don't need an Audio API. Modern JavaScript has the attack surface of Java and Flash combined and enabled by default, instead of hardening this against attacks browser are stuck in the same feature race they have been in since back when IE6 was still dominant browser.

> cache flush after every opcode

This would make JS outrageously slow, like hundreds of times slower than it is now.

You don't need it to be very fast just to respond dynamically to some event handlers and whatnot. (The basic scripting stuff JS was intended for.)

You also don't need modern wheels, when just pulling a simple carriage wheels were invented for.

Modern JS engines are not written for playing funny tricks with "window.status" or "window.title" as in 1990, but for more complex applications.

Not all uses are useful, but that's to subjective. As with wheels.