"Imagine being offered an option of spending the remainder of your natural life-span inside a virtual reality machine..."

This isn't what people want because people don't simply want "to be happy." They want X (where X could be something like "my children are healthy and happy"). Simply believing X, if it's not true, is not enough for most people, they desire for X to be ACTUALLY true. And since you can't affect the real world to bring your desires into existance from within the virtual reality machine, most people will reject this option.

"If the “meaning of life” is a meaningless concept, shouldn’t every reasonable person jump on this offer? Would you?"

I'm not sure I understand your objection... the whole point of the post was that "life is meaningless" is wrong. It is not a valid source of angst. It is merely something people say when they can't identify the correct problem to solve.