You know there might very well be some surviving descendants of the Forerunners outside the galaxy. We know that a self-isolating agrarian community of them were residing on Path-Kethona -- having been removed from the main Forerunner genetical strain for millions of years. It's possible that the descendants of Bornstellar and the lifeworkers who ventured out to live beyond the galaxy could have developed their own separate civilisation.I (not so) secretly want Bornsteallar to still be kicking around. I'd get a kick out of Chief coming across him at some point. I wonder if he looks like a mirror image of the Didact we fought in 4 or if the 100,000 years has aged him terribly without the right armour conditions.
The issue is that Bornstellar and the others decided among themselves to scatter across their own galaxy and abandon their armor so they'd grow old and die, allowing the galaxy to continue without them. Most of those surviving Forerunners were dropped on planets without a ship to leave on, so they definitely didn't leave, and I fully believe that Bornstellar and his wife would be content to grow old and die together... though the only thing that might throw a wrench into that plan would be their child, who could potentially find the Audacity and his father's armor... and if he had access to an AI, he could conceivably pull a "Doctor Who" and explore the galaxy as the last of his species, in a ship more advanced than anyone alive has ever seen.
That's not an outcome I'd root for, but it is more plausible than Bornstellar still being out there.