Java runtimes generate machine code just like V8 does, and there's no functional difference between "built-in" code and a plugin. A plugin is just a shared library.

At the time Java was being used in browsers, it had a lot of problems with that machine code generation. It took a long time for the JIT to kick in. The JITed x86 code it produced was poor. The startup time to load in the various libraries required by the applet tech was incredibly slow. It only started downloading the actual java code to run after the applet itself had finished loading everything. And more issues.

There problems were never truly about the idea being bad: it was the implementation that was terrible and was hindered even further by the Sun-Microsoft lawsuits, etc.

If everyone is on board with a technology and work to make it usable instead of 'feature complete', then it is very easy to make this work where Java applets failed. It's actually easy to make Java applets work where Java applets failed if you were to re-implement the idea with the correct focus and buy-in.

It's not technology - it is politics.