That is a very premature analysis. We do not know how much it will cost to harvest the gas resources in the hydrates, since the technology is only now being developed. Shale resources were long considered uneconomic until the day the technology and markets evolved enough to make them economic, which surprised most experts.
We also do not know what techniques will be used to drill the hydrates so no one can judge the environmental impact yet. I would argue that exploiting the hydrates offers many benefits over strip mining coal or tar sands. And we may find that we can sequester CO2 in the hydrates as well.
You could potentially be right that methane replaces coal and oil though, since it is far more abundant, non-toxic, and more versatile.
As to the potential for melting hydrates to impact the climate, only a small percentage are near enough the surface to be potentially unstable. The vast majority of hydate formations are below 500 meters of the ocean and plunge deeper from there and pose no threat of melting. But there are enormous quantities of hydrates, and some do pose a risk, particularly those in the permafrost. But I will leave it to the scientific community to judge the real risk, it is a new area of study with little in the way of conclusive answers.
Hydrocarbons are the foundation of industrial society, they are not going away, even as we need to find solutions to keep excessive quantities of carbon from accumulating in the atmosphere. The more natural gas we can use in place of coal and oil the better.