1). any business will start to decline at some point, including you(rs). So cost cutting is a proven solution. You are not buying a business, you deliver a service
2). the whole point of hiring subcontractors is to reduce risk (in my opinion). You are an expert they want&get when they need, probably with some strict clauses.
3). agree that the request is pretty left-handed
Secondly, about negotiations:
Everybody likes to feel special or more exactly that they "won" something. Why not make them happy by giving them a 2-3% (negotiate) and ask for this a contact-commitment of minimum 20/30/50 hours/month. That way you also minimize your risk of looking for work.
If you do have too much work however, then you have the upper-hand in negotiations. Play it now!