Monty did fork MySQL - but the name's still important
"Why doesn't Monty simple work on a fork, proclaim to the world "MontySQL is based on an old release of MySQL", and move on?"
He did that already, the product is called MariaDB. The point that Monty tries to get across is that the *name* MySQL is extremely important.
If Oracle makes a conscious effort to stop or even cripple "MySQL" in preference of their cash cow product, the mindshare that the product has gained will come apart. "MySQL" will become known as "the crippled product" and Oracle's sales folks will push people onto their other products.
Oracle has so far denied making any statement as to how they will support MySQL in the future, and much more importantly, only given vague promises as to the licensing under which MySQL will continue.
As I've written somewhere else: read the two articles on the helpmysql site. They are very enlightening.