Yes, that's kinda where I'm coming from in my way of thinking . . it may not necessarily mean certain death. But surviving feral colonies compete with natural wild birds for food and shelter and can end up causing lots of trouble as well. Look at all the feral muscovies that are a nuisance in so many places. Chances are some of those colonies began by someone who was unable to keep them from flying away.
Now I'm not saying these couple birds alone that belong to the op are going to make a big impact in the big scheme of things.. I can see they aren't just saying oh well, let them fly away if they want to or we wouldn't be having this discussion. But one way or another my advice would be, keep them from flying away either with a fence or enclosure or wing clipping. Whatever works.