Sure, so things slow down and it gets less usable. Understand that part, but how does that make it less secure.
When miners stop mining (because it is unprofitable and they are bleeding cash), mining hash power (activity) concentrates amongst a smaller pool of miners. Security is threatened when one miner accumulates too high a threshold of the total mining pool. That miner then has the power to control the blockchain (effecting "51%" attacks or other manipulations). Too much centralization of the mining pool = security risk.