Is the old model of the 80s/90s a good model where Microsoft is the big fish that either swallows everybody (for a price of course) or pushes them out of the pond altogether?

Contrast the "Microsoft model" with the "Apple model". If Xobni was a startup that added the same amount of value exclusively to Mail.app, then it would increase the overall diversity of applications for OSX (not to mention quality); granted, the fact that OSX isn't as popular as Windows might be factor with how Xobni/Mail.app might grow, but for the sake of illustrating the necessity of diversity when it comes to competition, let's put that on the side. On OSX, you have a number of excellent independent developers that add value to OSX thus increasing the number of things you can do with the OS. No doubt, there is absolutely nothing on OSX that you cannot do on Windows (forgive the double negative). But the fact that Microsoft is the big fish, independent developers need to be wary of the big fish by either getting bought out (good) or competing against them (most of the time VERY bad). Btw, a similar phenomenon is happening on the Web with Google, but we have yet to see how this is played out...

The "Microsoft model" is monolithic. As a result of the best case scenario playing out more often than not, if you show promise as an independent developer Microsoft will buy you out, thus decreasing the number of independent developers that have innovative ideas and increasing the number of crappy third-party software available (I will add that the fact that Windows is so ubiquitous might have something to do with this last opinion). Microsoft maintains dominance and every innovative idea out there eventually becomes a Microsoft department - slow, boring, and part of Microsoft. This is bad for Microsoft and ultimately bad for consumers.

OmniGroup is a real-life example of how the "Apple model" encourages independent developers such that they let them exist without the need for Apple to purchase them. Their suite of tools would really fit in well with some of Apple's offerings; but Apple doesn't buy them out because it is to Apple's benefit for OmniGroup to exist outside of Apple as an example of what you can do with OSX (as an independent developer), without having to worry about outside competition from Apple. Another example would be Delicious Monster. Apple benefits from the well being of outside developers as these developers add value in a way that encourages diversity. Small groups of devotees pop up around such outside developers and this ultimately trickles up to Apple in a positive way.

Xobni is an extremely well engineered piece of software, which probably required a high level of expertise to create; and this highlights another point: there is a high level of technical ability that is required to push the Windows platform in an overall innovative direction for independent developers. Whereas Apple makes it easier to develop/deliver a finished product, on Windows it is more difficult to compete against a Microsoft-made application. Simply put, the barriers to entry (development-wise) are significantly lowered on OSX than they are on Windows (I simply do not believe that OSX developers are more competent than Windows developers!). Microsoft certainly gives you tools to create some cool stuff if you have the development mojo/kungfu to use it well (as is exemplary with Xobni). Apple gives you tools that allow for easier/rapid development so that you can get your ideas to market gracefully.

That said, I wish the best for everybody involved in Xobni. No doubt getting bought out would be a big score for them and the Ycombinator folks! You guys have accomplished more than what I could hope for and all the best to you.