In 2003, RFC 3629 removed all 5 and 6 byte encodings, effectively limiting it to 4-bytes. Of course it could be expanded at any time, but that would be a significant change to established practice, and directly contradict the rationale in RFC 3629 (that because most people use 4 bytes in practice, allowing 5 and 6 constituted a security flaw).
You really want to measure life expectancy for people entering working age to avoid measuring infant mortality. Those numbers sound like they don't take that into account.
The point of his blog is to demonstrate how to make the realm of FU money easier to achieve. His major claim is that, since he owns his house and has chosen a relatively low-cost region to live in, his yearly expenses for his entire family are around $27,000. So agreed, for him $4k/month of free money would definitely qualify.
Because there are so many companies willing to provide comparable service that supply is nearly infinite. That offsets any feeling of what it's actually worth in productivity.
Source: the same Wikipedia article you linked.