We're not talking rocket scientists here. How would you go about doing those things? How would you be sure your method was effective. Building a bomb, and smuggling it into a public place is much simpler than dumping a sufficient quantity of poison into a reservoir (and you'd need truckloads).

It's terrorism. The higher the profile, the more the terror.
Tell that to China, Vietnam, Algeria, Iran and Israel. The current regimes did not used to be in charge. They weren't in charge 75 years ago, but terrorism happened, and now they are, largely because the West gave in, rather than their accomplishing any military conquest or political success. And increasing hatred and prejudice towards other Muslims retards assimilation, and will serve to radicalize more of them.

On what evidence do you base that assertion? When or where has it ever worked out like that?
And it either provokes the West into overreactions, or else causes the rest of the Muslim world to lose faith and respect for Western strength and institutions. Countries like Turkey, Iran and Iraq were very secular in our lifetimes, and expected to become moreso, and be the leading edge of civilization returning to the Middle East. And then radical terrorism, and what do we have now?
And some of those countries are part of the problem! Saudi Arabia is the homeland of most of the 9/11 hijackers. They couldn't keep their own population under control, and we're going to give them more? Why not crush Saudi Arabia and give it to the oil companies? What has Jordan done to show that they are capable of ruling half of Syria and suppressing dissent and radicalism?