Please provide evidence tying miners to operating also masternodes and please provide evidence that such combo is directly responsible for Dash price decline.
It's actually more incumbent on you to demonstrate that they're not since the protocol encourages hybrid mining as the most profitable configuration. So if you're trying to make the argument that for some reason this option is not being pursued and that miners & masternodes are distinct demographics than that would go against natural economic incentives and would be quite a bold claim.
Hybrid mining doesn't so much contribute to a "price decline" as a relative loss of marketcap compared with our 100$ mined competitors. The reason for that is that they don't have the option of reducing their aggregate difficulty by running masternodes which can mine at zero difficulty alongside their mining rigs.