If you are running a support forum which already has a members base than will it be wise to invest in a responsive design which is costly?
Its always a good idea to have a responsive design.
Since its support forum so it will be good. But here comes the question do you actually need it. If don't need it than why do it?
Responsive design is always good. But make sure that it doesn't effect your members user experience.
If you really want to establish your business field you need to provide a responsive design to benefit the users and to give good signal to the search engines.