Coming back to this after 4 years the observations I would make is that the world has taken a different path.
People needed a low cost alternative to Oracle, SQL Server, DB2, Sybase and to fill this gap PostGres and MySQL were born.[quote]
Presumably you are talking about a time after Sybase stpopped calling its product SQL Server and the name was used only by Microsoft, since you list SQL Server and Sybase as two separate things to which it would be an alternative. Even if you are talking about an earlier date than that, you have your history very wrong. The Postgres project was set up two years before Sybase released it's first odd bits of prototype, 3 years before first release of Sybase, and its first release was one 49 months before the first port of sybase 3.0 to Os/2 that MS had permission to do its own marketing of (as opposed to joint marketing with the other two owners of the product), that is about 6 years before MS first released a version that was more - only very slightly more - than a straight port of Sybase SQL Server. Sybase didn't change its product name from Sybase SQL Server to Sybase Adaptive Server Enterprise until 1997.
It's quite clear that Postgres wasn't a response to market pressure (Stonebraler had just released his commercial version of Ingres) but what Stonebreker said it was: a successor for Ingres in the academic world (in the commercial as opposed to academic world, Ingres was reasonably successful and still going strong last time I looked at it, in the early/mid 90s, but it's dropped back quite a bit since then) for people who wanted improved concurrency for OLTP and a decent user-defined type system and suport for Stonebrakers view of what domains are in the relational model.
For MySQL, the dates are a less impossible fit for your statement as MySQL 9 or 10 years later than Postgres. But when MySQL began both Ingres and Postgres were available as low cost alternatives to the big boys, so I doubt that the Swedes saw it that way rather than as a way to have open source rdbms project that conformed to their concepts or openness rather than to Californian concepts, or maybe just wanted something that was to a great extent mSQL compatible.
edit: while on historical inaccuracy, I can't resist quoting this from Steve's article:
What if we get to the point where a MySQL or PostgreSQL type knock-off that runs T-SQL comes along and implements 80% of what SQL Server 2005 has
Intersting idea: Postgres had snapshot Isolation before Microsoft SQL Server was first released, and had a very effective equivalent for what MS call CLR (much better, much easier to use, much more flexible right back then than IBM DB2's awful support of various languages for writing bits to be called from SQL was in 2000; it's hard to compare it as a whole thing with CLR because it's not based on a comon language runtime for a whole network application architecture in teh way that SQL CLR integration is, but in terms of allowing code to be built efficiently in a language other than SQL and then imported into and used in the database it is directly comparable). So things seem a bit more like SQL Server is a knock-off of Postgres that implements maybe 80% of its features....