Thanks for linking the article, JC... I really enjoyed it. I'm surprised thought balloons stuck around for as long as they did, honestly.
Well like he said, the thought balloon is a distinctly comic book idea. Its a way to tell the reader how a character feels about actions in a way you can't do in a movie or a book. It definitely had its usage. But the caption box just has more utility. You can put them anywhere on the page because they don't have a tail you have to connect. They can be utilized for dramatic effect in their placement and how they connect us in the heads of the characters.
Because of their "remove" from the narrative, they don't have to "pause" the action for a character to think all those complete thoughts. They don't have to comment on the current action in the panel, it could be about anything. They can show a juxtaposition between what's happening on the page and what the character really thinks, like that Black Panther page in the article. You can do that playful Alan Moore thing when he uses dialog in another scene to thematically link the action in another scene:
I use DKR and Watchmen because I always felt that was the major shifting point from thought balloons to narration captions. They were such huge influential works on just about everybody, even the really early work of Grant Morrison or old-school comic lover Mark Waid
Slowly but surely the Thought Balloon just got wiped out, to the point where Jason Aaron has to explain them to people in his new Thor series.