So far the word is that Dolphin 6.1x will not be upgradeable to Dolphin 7x. Now even if for some reason that it does happen to be upgradeable there will most likely be some complaints, problems, or other issues.
As I am sitting here thinking about this, one of the things that comes to mind is links, url's, addresses, or whatever you commonly refer to them as.
Say you have an established site that has been around for a while. You have many links or pages indexed by Google and the other search engines. Perhaps you even have many other sites linking to your site, or a specific page on your site.
Now since many changes are happening with Dolphin 7 when compared to Dolphin 6.1x many of these links or addresses would change.
Many of your pages that Google has indexed will no longer exist. Same with any other site that might happen to link to any of your inner pages. They could no longer exist as many pages in Dolphin 7 will have a much different address.
So if you have spent a lot of time getting traffic, members, good Google indexing and ranking, you could potentially lose a lot of this hard work.
If you have a brand new site or one that doesn't have a lot of content yet, you probably won't be out that much.
You could possibly use some redirecting in .htaccess to the new page or address in Dolphin 7, but if you have a lot of pages this could be very time consuming.