I fail to see a problem with evolution. Species responds to pressures placed on it by the environment, which causes individuals who are physically better able to handle the new environment to survive easier. If there is no pressure, there is no real selection for a new species. The shark, on one hand, has changed quite a bit in 65 million years, while appearing to retain some of the characteristics that made it a "shark". See Discovery.com for a few fanciful graphic renderings of existing shark fossils.
As far as the coelocanth, the thousand coelocanths remaining after all of this time are but one species that closely resembles that of one of the 120 species gleaned from fossils we have 400 million years ago. It doesn't sound particularly successful to me. It sounds like the coelocanth has found its tiny niche, and is successful there. Link www.dinofish.com
I don't know much about blowfish, so I must defer there.
My point remains that since since evolution is a statistical process, and only responds to selective pressures in the environment, therefore a species will not change if there is no reason to change. Evolution does not dictate change for change's sake.
Here's the problem with the above explanation: for change to occur you need mutations. Mutations are necessarily random and occur all the time. Most mutations are bad, but if a favorable mutation occurs - even in a successful species, why would it not be adopted anyway? There are many adaptations that can be helpful even in a successful species - a smarter brain would help any species for example. So why would such a mutation be rejected? None at all. Yet you say that some species are too well adapted to change. Clearly that is not true, there are always improvements possible.
There is also another problem with your explanation - why would a successful species change? There is no reason at all. Perhaps the most successful species ever are bacteria. They have been around since life first started. They are everywhere, even inside our own bodies. Why would they bother to change? There is no need for it, they are now and have always been highly successful. They have survived for hundreds of millions of years. Yet if evolution is true, we must believe that this most highly successful species chose a path more fraught with problems than the one it had already achieved.
In other words, you cannot have it both ways.