The one thing I tend to say about any service or product is whether it provides you with the necessary services or features you or your community needs. We can go around and around wondering if some unknown feature will be a godsend for our community in the future, and we will end up not building a community at all. Will IC6 be cloud-only? I don't know, and as has been mentioned a few times already, that is roughly 10 years from now. Who knows how your community will look then or if it will even be around? I know for myself that I'm perfectly content with IC4 and even more so with IC5. If my community is still around in the next decade and the next version of IC isn't something I could use or want, I can make the decision then to migrate to a different service or infrastructure.
Basically, make your decisions for your community's needs on the information in the present. Have a plan for a possible migration in the future if IC doesn't meet your needs in the future.