In theory, you could take the core Discourse PostgreSQL database and the Discourse rails application, and design your own SPA UI, from scratch.
Please note that this would not be a "replacement" but a ground up creation of a forum application using the current Discourse database and the Rails application.
Rails is the core of the MVC application, and PostgresSQL is the database. In theory, if you wanted to, @Harsha1, you could start with this and build your own SPA on top.
In fact, Discourse currently serves only the Rails application to search bots like Google; so in fact the Rails application does indeed work without the SPA - but is is very bare bones, nothing like the full SPA!
As other have said, you cannot simply "replace Ember.js" but you could build another SPA UI from scratch using the Rails MVC core with the current PostgreSQL DB.
Note that it might take you many, many years of SPA work to get to the rich feature set of the current Discourse SPA. Not sure if that is a good way to spend your years 
Ember is core to the Discourse SPA and cannot be simply "replaced" as mentioned.
Hope this helps.
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