But if an exchange goes through a hard period of time and shuts downs then ultimately that token will come to its end because there is no community other than that exchange supporting the token simply because it has no usage outside the exchange.
According to me exchanges should promote the usage of their coins/tokens outside the exchange to build a community in support of it. If your community consists of huge individual entities who believe you then a great community can be formed. Otherwise lack of applications would lead these things to disaster.
There's truth to your post, each token created that are centralized depends so much on their platform if the platform fails to generate support or the exchanges is shutdown, there is no usage for the token anymore, so if you are going to buy a token, be sure that the platform or project attach to it is solid and has a lot of support.