I think we're put on this earth to create things of great lasting value that are good for others around us. Many things in our lives will change, but I believe this one is a constant guiding light.
Support him? By bailing out? Was it easy for your cofounder to replace your skillset? Would love to hear the whole story if you don't mind, it could help help others who are in similar situations. It's sad that cofounders who once created something together are not able to find a common understanding in later stages. Sad that both parties reached that point where they have to utter the threat either my way or I leave. WTF. I know this is common and probably the reason why most startups fail but just leaving (or maybe the true story was different) sounds a bit strange. However, maybe it was the best decision for the organization.
Support him BEFORE he bailed out. Ie. he supported the VC thing, although he didn't like the direction.
As for the "skill-set", it's not as if building a web service, and such as simple one as posterous (or twitter, or tumblr) is rocket science.
Scaling it is a little harder, but still, thousands of companies and websites do it on their own. So what What ireplaceable skillset?
And now he wants to work on something like Posterous again. His priorities don't seem so shifty.
It's nice to see you acknowledge that things were rocky with Posterous but you are now moving forwards in a new direction based on your past learnings.
After all, isn't that one of the great things about being human? We can all learn from our past mistakes and strive to better than the person we were yesterday.
But on the other hand, you did accept VC funding for Posterous (I'm assuming nobody was holding a gun to your head and forced you to do it), and for whatever reason you then left and moved onto other projects.
Now you are saying you will never sell out, will never lose focus and will stick around forever. Isn't that like a politician promising they won't ever raise taxes? "Ever" is just a really strong word. Life happens. Priorities change. Heck, most marriages don't even last forever.
That said...
I believe you. I think you're telling the truth about how you actually feel right now and what your plans for the business are right now. It's just that right now only lasts an instant.