No, the reason they can do so well in freshwater for so long is because it's a part of their life cycle. However, the cycle also includes brackish and marine. If kept in freshwater for too long, they will suffer. Many have lost their gobies to red tumors that spread and grow all over the fish's body until it dies.
Their main habitat is estuaries. Estuaries are brackish.
Here's a quote from Neale Monks. Neale is a very knowledgable person and "I've learnd a lot from the things he's posted on forums around the web.
Your Violet Goby/Dragon Goby -- Gobioides broussonnetii -- is indeed a brackish water fish. While they do occur in freshwater in the wild, they are rarely far from the sea, and in aquaria seem to last only a year or two in freshwater conditions."
Here's another, in response to somebody saying that the "salt factor" is up for debate.
"Only debated by the ignorant; these are estuarine fish, period."