While it's true that CAN happen, it doesn't mean it happens often enough to disrupt JIT compiling the code. It does mean there is an invalidation when code paths are modified.

JRuby is a thing and it runs on the JVM as more than a simple interpreter. The JVM does have to deal with this exact problem. So you could actually say Ruby can already run as fast as the JVM because it runs on it, what it cannot do is run as fast as a static language because, at the very least, it has to deal with additional checks that things haven't changed.