The reason for the Haessler piano is quite simple. After the de-nationalization of the Blüthner factory the world markets started to open up to the firm which was behind the Iron Curtain until 1990. Demand grew for fine European pianos, at the same time as demand grew for instruments with a more modern voice. A Blüthner piano has a particular voice, and it isn't "modern".
The Haessler piano fills the company's need for a modern sounding instrument in the high-end market (I'd call a retail price of almost 30k for an upright high-end even though I know there is higher end!), and they've achieved this by streamlining some of the manufacturing processes. The pianos are all loop strung, the frame isn't finished to quite as high a standard, the soundboard system is more conventional, but it's all still done to the Blüthner quality standard. The actions are Renner and they're built to the highest standards of quality as you'd expect in that price range. Terminaldegree is right, there is a rather nice 186cm grand from Haessler, and some of the large uprights are beautiful. A lot of the case parts like the music rack, pedals, front pillars of the uprights, fallboards etc are exactly the same as you find on Blüthner pianos.
Haessler is actually the surname of the family now, it's double barreled "Blüthner-Haessler" and so technically if they wanted they could change the name of all the pianos to "Haessler", and it's a product they believe in. Some of the PH Grands, although they were branded Blüthner were actually built on Haessler plates, although they've changed that now and they're built on Blüthner plates, but that doesn't matter, since both pianos are built by the same people in the same factory with a lot of the same materials. In that respect, a Haessler piano *is* a Blüthner piano but it's just branded differently. That sets it apart from Hoffman, which is made in a different factory, and Boston, which is made by a different company. Not that there's anything wrong with Hoffman or Boston pianos, but they're not high-end instruments even if they are very good.
If you like the piano and you're happy with the price, but your concern is the quality in the long term, you've nothing to worry about. I hope that clears everything up for you!