Any serious user of MySQL is already using the InnoDB storage engine. (The
default storage engine, MyISAM, is non-transactional, supports only table-
level locks, and is overall totally unsuitable for most serious load
patterns.) InnoDB is GPL+proprietary software owned by Innobase -- which
was acquired by Oracle in 2005.
Oracle could already have crippled MySQL by killing development of its only
serious storage engine. It didn't. It funded big improvements of InnoDB,
including major performance and scalability improvements. There's no
reason to think it will turn around and do something different to MySQL
itself. To the contrary, maybe it will now make InnoDB the default storage
engine as it deserves; MyISAM only really makes sense for special-purpose
loads, and it's probably been the default because MySQL didn't own InnoDB.
Oracle is a company whose goal is to make money. It currently only has a
super-high-end database product. MySQL expands its range of offerings by
giving it the leading low-end product too. If it encourages MySQL to grow,
it will own both the top and bottom of the market, giving it more leverage
to squeeze out competitors like MSSQL in the middle. If it kills MySQL, it
loses a valuable asset; the low end will go to a fork or PostgreSQL. It
has no rational reason to do this. MySQL doesn't seriously compete with
Oracle. Not to mention Oracle's made legally binding commitments that it
won't kill MySQL, at least not soon.
I'm much more interested in what will happen to Solaris. Oracle has been a
prime backer of btrfs, for instance, whose only real goal is to give Linux
a ZFS-like filesystem. Now Oracle will own ZFS. Will it see any reason to
keep its assets incompatibly licensed, or will it GPL Solaris and let ZFS
(et al.) get ported to Linux? btrfs won't be as stable and featureful as
ZFS for several years yet.