Lots of shows do great in their "heyday".
TV shows generally continue airing if revenue generated is significantly greater than revenue spent. If Johnny Test was accomplishing this for Cartoon Network then that is really all that matters. It doesn't always matter how a show is doing compared to competition on other channels. It is highly unlikely that Johnny Test would ever get more viewers than Spongebob or Phineas and Ferb so I wouldn't think Cartoon Network was looking to get those numbers with Johnny Test.
TV shows generally continue airing if revenue generated is significantly greater than revenue spent. If Johnny Test was accomplishing this for Cartoon Network then that is really all that matters. It doesn't always matter how a show is doing compared to competition on other channels. It is highly unlikely that Johnny Test would ever get more viewers than Spongebob or Phineas and Ferb so I wouldn't think Cartoon Network was looking to get those numbers with Johnny Test.