Things can turn around quickly in the NBA.  You're right that a lot of people in the media seem really excited to throw dirt on the Celtics.  It's a predictable backlash to the Celts being a golden team just a year ago and then having the sort of season of discord and disappointment that is usually reserved for franchises that Celtics fans like to deride and dismiss.
I definitely think that most folks in the media have been too quick to dismiss Tatum and Brown just because they didn't make a leap to stardom this year.  Sure, they had a disappointing season.  They didn't improve like we hoped.  That doesn't mean that their chances of coming close to reaching the very lofty expectations people had for them after last season are suddenly zero.
I agree that it feels kind of crazy that the Celts could be in a drastically worse position after it seemed like they were set up to be a top contender for the next 5-10 years.  That's just a feature of Danny's strategy.  Kyrie and Horford could leave in free agency, and the Celts will get nothing in return.  There is no guarantee that any team will want to trade a superstar player to the Celts for what the Celtics have to offer.
Still, even if all of the free agents leave and the Celts are forced to use all of their draft picks, the Celts will be in a much better position than many NBA franchises.  They will still have a proven coach, a smart GM, and competent ownership.  They will have a young team with plenty of potential, and a reasonably clean cap sheet.  They will have plenty of ways to improve moving forward.
I'm generally pretty pessimistic, but I think the chances are very good that next year's Celtics team will be much more enjoyable to watch and follow on a nightly basis than this year's team.  Super low bar, but still that should be a comfort as we wait to find out what will happen.