There is no way in the world of software that anyone can predict ten years out whether the solution you build today will still be viable.  You can guarantee that SQL Server and even the OS you run on won't be supported then.

Jeff

1. Higher versions have some overlap with the older ones. You don't have to run the latest version to get support. 10 years from now though, I doubt if anyone will remember how to help you with version 4.x.
2. You can set a "check for updates" setting that will pop up a new version icon on your admin console when one is ready, same with the core modules, if you want.
3. Support is free through the forum or other's web sites and blogs. Paid support can be had through a number of vendors. I get about 80% of my support right here. Most paid support is by packaged hours per month/ quarter. About $90 hour I think for most services.
4. I spend a large portion of my time researching issues, testing, evaluating and fixing as opposed to forging ahead building the site. But that's part of the trade off when you use any open source application and taking advantage of all the 3rd party modules out there. You're basically doing what you would be shelling out big $$ for someone else to do in a cms company. 
5. Impossible to tell the future. With such a rapidly growing base of DNN users I would be very suprised if those steering the DNN ship put out a release that did not have a solid upgrade path. In fact, I have found upgrades to be getting a little easier over the past year that I have been doing them. I think all the core modules are held accountable to some pretty strict guidelines from what I have read.

I basically accept the fact that there is no 'hot line' to call when things go really bad (they only did once for me). People on the forum are awesome at helping you. And with a lot of reading up on DNN and some good backup planning the risk is minimal. DNN just has way too much to offer. Hope that helps.
Mike