Runtime checks don't stop you from doing heavy data processing or having microsecond-level response times. It seems like your objections fall into two categories. One is caused by it running in ring 3, which can be solved by correcting "anything" to "any user program". The other is moderate performance limitations that don't stop it from running full video game engines. Those may not be ideal but they won't stop you from running anything. Video games are among the most demanding things you can do in most ways. The single core of today is better than the multi core of a few years ago, and nobody claimed that those couldn't run all programs.