There's nothing that prevents any artist or team from creating and marketing their own figures, for whatever program they want it to work in. If the quality is there, then there will be a customer base for it.

It's my understanding that the only real issue currently preventing G3 from functioning in Poser is the dual quaternion rigging. This has become an industry standard form of rigging/skinning, (Maya, Max, etc), so it's not something that DAZ has come up with on their own, the way it was with G1 and 2. It's up to Poser's developers to adapt to that rigging if they want their user's content to be cross-platform compatible, and for other developer's content to function in Poser. 

As for 'Rosity creating their own figure, that was already tried about 10 years ago and didn't work out very well, for various reasons. But it would be interesting to see if they're up to attempting it again though. It might be a good idea. And there's no reason they couldn't develop their own figure as a sort of mascot for the site, and continue selling content for other figures. With proper scripting, any content can be adapted to any figure.

Hasn't RDNA created at least a few of their own figures in the past? Or at least heavily supported figures that are exclusive to their site. I thought they were involved in Michelle's development, and more recently the Star figure.

As for vendor vs customer support on new figures, it's really more of a 50-50 split. Customers can't buy content that vendors don't make, but if enough of them don't buy the content that vendors do make for a figure, then it doesn't give them much motivation to make more content for that figure. In the end, vendors are in it for the money and are going to make content for the figures that sell the most. So really, it all comes back around to the figure's creator being responsible - at least in the beginning - for producing the bulk of content for his/her/their figure, if they want that figure to keep going. At this point, any figure that isn't compatible with a lot of the existing content is going to struggle in support for the first while. If you make a figure you have to be willing to invest a lot of effort into supporting it yourself, with quality content that people want to use.