Discovering brand new things at a constant rate is probably not a realistic expectation. The 20th century has been an extraordinary journey for astronomy that cannot possibly continue indefinitely (same with other parts of fundamental science). We went from barely a sense what our galaxy is about, to discovering an extraordinary number of exotic astronomical objects by progressively expanding the window into radio, x-ray, gamma rays, gr waves etc.
But I don't think its true that future astronomers / space telescopes are condemned to boring hires pictures of known stuff. The excitement is not in the visual impression but in what it might tell us about the universe. In the end what matters is whether we understand what is happening (e.g., in this instance how planetary systems like our very own are forming). So while the visual impression might not alter much, the quantitative detail may be fixing the physics or even prompting completely new theories. Remember sometimes even the stuff you don't see can bring about a revolution (dark matter, dark energy).
But I don't think its true that future astronomers / space telescopes are condemned to boring hires pictures of known stuff. The excitement is not in the visual impression but in what it might tell us about the universe. In the end what matters is whether we understand what is happening (e.g., in this instance how planetary systems like our very own are forming). So while the visual impression might not alter much, the quantitative detail may be fixing the physics or even prompting completely new theories. Remember sometimes even the stuff you don't see can bring about a revolution (dark matter, dark energy).