Neil, thank you for your post. It is very interesting to explore new names for me.
German composers of course developed symphonic genre in music. The genre was taken by composers of different nationalities.
There was very interesting program about symphony and how different composers developed the form. Every new generation re-evaluate composers. For example, Sibelius was not played as much and now his compositions are played and his music influences other composers.
I think symphonic genre even influenced opera and how composers wrote them. Wagner’s operas are composed through with leitmotivs and not separate numbers.
Composers still write symphonies now.
Here is a program that was very interesting about development of genre and re-evaluations that are made now.
http://www.bbc.co.uk/programmes/b017chz0