With desktop applications you can use the same language/code on both the client and the server side. That did not lead to decentralization. Well, that isn't cross platform you might say. However, Java mostly covers that and still it did not happen. There are also numerous tools that allow you to write in one language (Java with GWT, some lisp one I cannot name) that runs on the server side that automatically generates equivalent javascript for the client.
There is no reason (and you provide no argument) that using the same code on the client/server results in decentralization. Furthermore, we have the fact that it already exists and still did not lead to decentralization.
That will eventually lead to decentralised applications, but in the mean time will simply lay the groundwork for it whilst people continue with the centralised model.
There will always be a need for centralised systems, the world is just not going to simply change their entire toolset and paradigm overnight. We must make centralised apps use pretty much the same technologies as decentralised ones first, then coax developers over.