Why do you need all manpower for the existing products?
Was the staff reduced after completion? - Hire new guys
Are the products -ahem- very defective? - Fix your process, and fix your bugs first (sorry)
Did you develop many products, with more and more time going into maintenance, and now no development is left? - Cancel support for old products, hire new guys, or make clients pay dearly for maintenance (i.e. let them cancel support)
Are you frantically adding new features to the products? - sorry, you are doing new stuff. New features need to be balanced
Does the comany require more "R"? Many R&D companies end up M&F - Maintenance and Features. Are the other guys happy? If so, maybe you need to look for a position better suited to you.
I am not sure about your position - are you doing inhouse development, or is your company selling the software? In any case, there must be some room for new development, to remain healthy. Make clear to your management that, if the job consists purely of maintenance, the best people will leave.
Don't expect to much, though - especially if you are on inhouse development. It is estimated that 80% of all positions are maintenance (I wish I could find a reference).