You are right (and that was in 2016). Hecking at least was fired after the season, not during it, so he exactly had one season there, but that's it. Every year a new coach tries to get them back to the first league, every year they fail. They really seem to be unable to build something. Giving a coach a year to settle and to develop his team, and then giving it a real challenge in the second season is something they spectacularly fail to do so.
And I do think they were absolutely right when they fired every single one of their coaches and did see no further improvement in the team, the funny thing is when it happens so often, can you really always blame the coach? They all seem to have lost the dressing room and the team always needed a new impulse (which was never enough), so why that happens is the real big question for Hamburg.
I wouldn't say that they aren't building anything. Their points total is relatively stagnant at 56 -> 54 -> 58, but their GD suggests steady improvement at 45:42 -> 62:46 -> 71:44
Every year they are at or close to the top until they drop out of the promotion zone 2-3 games before the end, which suggests their problem isn't squad building or bad coaching in general, but the team cracking under the pressure of being a club like Hamburg in the second division. It's a bit like Schalke or Leverkusen when they got close to winning the league, the weight of winning their first title (in 50 or so years) and the memories of failed attempts just completely wrecked them on the finishing line.
So now they are in a spot where they basically have to try a new coach every year until they get lucky or they find a coach who is able to moderate that pressure or create so much of a cushion that they can't let it slip.
This actually reminds me of a funny article I've read a couple of days ago, though (un-)fortunately it's only SportBild.
They claim that Schalke's directors are analyzing Hamburg's games to avoid making the same mistakes. They have supposedly made out three key points:
1) Hamburg dropped a lot of points late into the game, so Schalke are looking for a workhorse that can close gaps even late into the game (Latza is supposedly such a signing).
2) This could be straight from the 90s and would be Schalke in a nutshell: Hamburg only fouled 8 times when they dropped it in Heidenheim under Hecking and only 7 times in a similar situation under Wolf. Therefore Schalke want to sign players who fight back.
3) It's important to have a goal getter, so they signed Terodde.
https://sportbild.bild.de/bundeslig...n-vom-hsv-versagen-lernen-76469028.sport.html