Theoretical here of course, but that might make sense from a developer standpoint. I know they wanted a hard to figure mechanic but unless there's a dupe glitch (or MM) to test with, I don't think there'd ever be consistent ability to drop boss souls. Unless they predicted exploits to dupe items I don't think even From would be cruel enough to have Boss souls be a trigger (or more accurately a different trigger).
Is there any information on leaving vagrants alone? I thought I read or heard something about them getting stronger over time. If that's accurate then that could account for the idea of items gaining value. However, this opens the door to further variables. Especially if the vagrants drift if left alone. The chances of spawning a drift, leading to spawning a vagrant for yourself to letting it alone and spawning a stronger vagrant which in turn leads to spawning the strongest vagrant back to your world seems unnecessarily convoluted.
EDIT: Sorry, if I had more time to play I'd be running tests too but I have very limited windows these days.
Is there any information on leaving vagrants alone? I thought I read or heard something about them getting stronger over time. If that's accurate then that could account for the idea of items gaining value. However, this opens the door to further variables. Especially if the vagrants drift if left alone. The chances of spawning a drift, leading to spawning a vagrant for yourself to letting it alone and spawning a stronger vagrant which in turn leads to spawning the strongest vagrant back to your world seems unnecessarily convoluted.
EDIT: Sorry, if I had more time to play I'd be running tests too but I have very limited windows these days.