I agree with you in many ways, but I can explain some stuff around why the new Hammarby stadium was built.
Since the 80´s Hammarby fans have worked for a upgrading of the capacity of Söderstadion. Many ideas to build curves and new etages at Söderstadion have passed the politicals but nothing happened, the only thing we got was a new stand at the short side under the start 0f 90´s. There was no good expansion options and it is still not. Today Söderstadion takes 12.500 and that´s usually enough when we play in 2nd division, but when we are in 1st divison the stadium is full almost every game and with a bigger stadium we would attract much more people.
The biggest reason why we supporters wanted to rebuild Söderstadion was however not the capacity, it was because we´re not allowed to play high-risk matches (especially derbies) and Europa games. We want to play all our matches at our home stadium.
The time showed then that it was no chance to increase the capacity at Söderstadion and that´s why the politicans together with our club and the operator Anchultz decided to build a new stadium 500 meters away, in the "slaughterhouse area". Of course we wanted a stadium for around 20.000 but it would be 30.000.
We will get 6.000 standing places and do like the "german model" where is it a concrete-stand that can change to seats (for europa games). Why I talk about derbies and europa games is because we probably will play in 1st division and be a great team in a few years.
We´re working to get so much of our culture from Söderstadion with us to the new stadium and Bajen Fans, the supporter group, has aquired an important voice that all involved is listening to.