The 451 Group just published a "glass half empty" assessment of the open-source database market. One big takeaway? Open-source databases are widely used, but not yet deeply used.
One of the key findings is that open source software has had a superficial impact on the enterprise database market in that adoption has been widespread but shallow. While open source databases have been widely deployed for Web-tier applications, there has been minimal adoption in the enterprise application tier, and adoption for enterprise applications is at this time limited to certain specific application workloads.
To which I'd respond, yes, but you've got to start somewhere, and it's impressive to be able to start with some of the most demanding customers on the planet (Google, Yahoo!, Digg, etc.). It's a bit like programming platforms in the enterprise: Java and .Net still reign. Importantly, however, there has been a huge uptick in adoption of PHP, Ruby, etc.
Perhaps open-source is growing new markets instead of just cannibalizing old ones? That, to me, is much more interesting than a business of pilfering someone else's lunch money.